Fissile neutron detector

ABSTRACT

A fissile neutron detection system includes a neutron moderator and a neutron detector disposed proximate such that a majority of the surface area of the neutron moderator is disposed proximate the neutron detector. Fissile neutrons impinge upon and enter the neutron moderator where the energy level of the fissile neutron is reduced to that of a thermal neutron. The thermal neutron may exit the moderator in any direction. Maximizing the surface area of the neutron moderator that is proximate the neutron detector beneficially improves the reliability and accuracy of the fissile neutron detection system by increasing the percentage of thermal neutrons that exit the neutron moderator and enter the neutron detector.

TECHNICAL FIELD

The present disclosure relates to the detection of high energy neutronssuch as those emitted by plutonium and highly enriched uranium.

BACKGROUND

Governments mobilize radiation detectors to stop the illicit movement ofnuclear material such as plutonium and uranium. Previous approaches toneutron detection have relied upon an isotope of helium gas, helium 3 or³He, a limited resource generated during the construction and/ordecommissioning of nuclear weapons which is already showing signs of aglobal short supply. Due to increasing ³He shortages and the resultingincrease in associated costs, neutron detectors utilizing ³He cannot beeconomically deployed at scales. Efforts to develop replacementtechnologies have been initiated however, none of these efforts haveproduced a cost effective, scalable solution.

The lack of scalable technology has limited the evolution of existingsystems to meet evolving threats. Specifically, current modeling effortsshow that the deployment of a large, networked array of detectiontechnologies—where the detectors are placed at potential points ofattack, material source locations, and discreetly at randomized pointsof transportation pathways-will lead to the greatest increase of overallsecurity against nuclear threats.

Plutonium and highly enriched uranium (HEU)—materials that can be usedin a nuclear weapon—emit both gamma rays and neutrons. After the attackson Sep. 11, 2001, the U.S. government sought to strengthen borderdefenses against smuggled Special Nuclear Materials (SNM). To detectSNM, Federal, state, and local governments deployed detection unitsusing on 3He gas in proportional counters wrapped in high-densitypolyethylene (HDPE)—a technology pulled from physics laboratories andthe nuclear power industry. Polyvinyltoluene (PVT) plastics hooked up tophotomultiplier tubes (PMT), pulled from the scrap-steel industry, wereused to detect gamma rays emitted by HEU, as well as other dangerousradioactive sources that could be used to create a radiologicaldispersive device. Handheld devices, which have better gamma ray energyresolution than PVT, supported the main scanning capabilities of theselarger ³He and PVT detectors.

This initial detection capability had challenges. The initial deploymentof neutron detectors severely depleted the limited stockpile of ³He,driving costs sky-high and limiting scalability of deployment. Equallyproblematic were the number of false positive alarms that were due tothe poor energy resolution of PVT, increasing overall scanning times andlimited the usability of the systems. Multiple government R&D programsover the past ten years have invested in ³He alternatives for neutrondetection, as well as improved energy resolution gamma ray detectionunits. However, while some alternative materials have emerged, none ofthe R&D programs succeeded in reducing the cost of these systems.Furthermore, low-cost, large-surface-area gamma ray detectors have notbeen realized at any reasonable price point, with most of the work stillfocusing on smaller surface area (around 200 cm² active regions)devices. The original desire to replace neutron detectors with largesurface high-energy-resolution gamma-ray detection (which could detectthe gamma ray signals from both Pu and HEU) has therefore faded. Giventhat 1.2 million kilograms of Pu has been produced since World War II,and its key signature is neutron emission, neutron detection is nowconsidered a non-negotiable component of threat detection capability.

For these reasons, new neutron detection solutions are needed. Thesolution should:

-   -   Be low cost and independent of ³He. This will enable scalable,        affordable solutions;    -   Have low probability for gamma-ray induced false positives by        having high gamma ray rejection and secondary gamma ray isotope        identification system;    -   Be rugged and long lived for compatibility with military CONOPS;        and    -   Hit metrics of capture area and efficiency to detect the desired        threats will be a major advance in the overall reduction of        nuclear threats.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of various embodiments of the claimed subjectmatter will become apparent as the following Detailed Descriptionproceeds, and upon reference to the Drawings, wherein like numeralsdesignate like parts, and in which:

FIG. 1A is a front elevation view of an illustrative fissile neutrondetector, in accordance with at least one embodiment of the presentdisclosure;

FIG. 1B is a partial cross sectional view of the illustrative fissileneutron detector depicted in FIG. 1A, in accordance with at least oneembodiment of the present disclosure;

FIG. 1C is a side elevation view of the illustrative fissile neutrondetector depicted in FIG. 1A, in accordance with at least one embodimentof the present disclosure;

FIG. 1D is a partial cross-sectional view of the illustrative fissileneutron detector depicted in FIG. 1C, in accordance with at least oneembodiment of the present disclosure;

FIG. 1E is a partial sectional view of an illustrative isolator throughwhich an electrode is introduced to an interior chamber in the neutrondetector, in accordance with at least one embodiment of the presentdisclosure;

FIG. 2A is a cross-sectional view of an illustrative fissile neutrondetector that includes two neutron detectors with an interposed neutronmoderator, in accordance with at least one embodiment of the presentdisclosure;

FIG. 2B is a cross-sectional view of an illustrative fissile neutrondetector that includes three neutron detectors with interposed neutronmoderators, in accordance with at least one embodiment of the presentdisclosure;

FIG. 3 is a chart depicting neutron detector performance as a functionof operation parameters for an illustrative fissile neutron detectorhaving two neutron detectors separated by an interposed moderator suchas depicted in FIG. 2A, in accordance with at least one embodiment ofthe present disclosure;

FIG. 4 is a chart depicting neutron detector performance as a functionof operation parameters for an illustrative fissile neutron detectorhaving three neutron detectors separated by interposed moderators suchas depicted in FIG. 2B, in accordance with at least one embodiment ofthe present disclosure;

FIG. 5 is a chart depicting neutron detector performance as a functionof detection sheet placement within the detector, in accordance with atleast one embodiment of the present disclosure;

FIG. 6A is a perspective exploded view of an illustrative neutrondetector, in accordance with at least one embodiment of the presentdisclosure;

FIG. 6B is a cut away plan view of the illustrative neutron detectordepicted in FIG. 6A, in accordance with at least one embodiment of thepresent disclosure;

FIG. 6C is a sectional front elevation view of the illustrative neutrondetector depicted in FIG. 6A, in accordance with at least one embodimentof the present disclosure;

FIG. 6D is a sectional side elevation view of the illustrative neutrondetector depicted in FIG. 6A, in accordance with at least one embodimentof the present disclosure;

FIG. 7A is a perspective view of an illustrative neutron detector havinga number of detector electrodes, in accordance with at least oneembodiment of the present disclosure;

FIG. 7B is a partial elevation view of a neutron detector electrode thatmay be used in the illustrative neutron detector depicted in FIG. 7A, inaccordance with at least one embodiment of the present disclosure;

FIG. 8 is a high-level flow diagram of a method of detecting fissileneutrons using a fissile neutron detector, in accordance with at leastone embodiment of the present disclosure;

FIG. 9 is a high-level flow diagram of a method of detecting fissileneutrons using a fissile neutron detector that includes at least oneactive material in the form of a solid sheet or layer, in accordancewith at least one embodiment of the present disclosure;

FIG. 10 is a high-level flow diagram of a method of detecting fissileneutrons using a fissile neutron detector that includes at least oneactive material in the form of a gas, in accordance with at least oneembodiment of the present disclosure;

FIG. 11A is a cross sectional view of an illustrative neutron detectorand neutron moderator arrangement that may be used in one implementationof a fissile neutron detection system, in accordance with at least oneembodiment of the present disclosure;

FIG. 11B is a cross sectional view of another illustrative neutrondetector and neutron moderator arrangement that may be used in oneimplementation of a fissile neutron detection system, in accordance withat least one embodiment of the present disclosure;

FIG. 11C is a cross sectional view of another illustrative neutrondetector and neutron moderator arrangement that may be used in oneimplementation of a fissile neutron detection system, in accordance withat least one embodiment of the present disclosure;

FIG. 11D is a cross sectional view of another illustrative neutrondetector and neutron moderator arrangement that may be used in oneimplementation of a fissile neutron detection system, in accordance withat least one embodiment of the present disclosure;

FIG. 11E is a cross sectional view of another illustrative neutrondetector and neutron moderator arrangement that may be used in oneimplementation of a fissile neutron detection system, in accordance withat least one embodiment of the present disclosure;

FIG. 11F is a cross sectional view of another illustrative neutrondetector and neutron moderator arrangement that may be used in oneimplementation of a fissile neutron detection system, in accordance withat least one embodiment of the present disclosure;

FIG. 11G is a cross sectional view of another illustrative neutrondetector and neutron moderator arrangement that may be used in oneimplementation of a fissile neutron detection system, in accordance withat least one embodiment of the present disclosure;

FIG. 11H is a cross sectional view of another illustrative neutrondetector and neutron moderator arrangement that may be used in oneimplementation of a fissile neutron detection system, in accordance withat least one embodiment of the present disclosure;

FIG. 11I is a cross sectional view of another illustrative neutrondetector and neutron moderator arrangement that may be used in oneimplementation of a fissile neutron detection system, in accordance withat least one embodiment of the present disclosure;

FIG. 11J is a cross sectional view of another illustrative neutrondetector and neutron moderator arrangement that may be used in oneimplementation of a fissile neutron detection system, in accordance withat least one embodiment of the present disclosure;

FIG. 11K is a cross sectional view of another illustrative neutrondetector and neutron moderator arrangement that may be used in oneimplementation of a fissile neutron detection system, in accordance withat least one embodiment of the present disclosure;

FIG. 12A is an exploded view of another illustrative neutron detectorthat may be used in one or more implementations of a fissile neutrondetection system, in accordance with at least one embodiment of thepresent disclosure;

FIG. 12B is an assembly drawing of the illustrative neutron detectordepicted in FIG. 12A, in accordance with at least one embodiment of thepresent disclosure;

FIG. 12C is a detail drawing depicting an electrode connection devicesfor use with the illustrative neutron detector depicted in FIGS. 12A and12B, in accordance with at least one embodiment of the presentdisclosure;

FIG. 12D is a detail drawing depicting another electrode connectiondevice for use with the illustrative neutron detector depicted in FIGS.12A and 12B, in accordance with at least one embodiment of the presentdisclosure;

FIG. 12E is a detail drawing depicting another electrode connectiondevices for use with the illustrative neutron detector depicted in FIGS.12A and 12B, in accordance with at least one embodiment of the presentdisclosure;

FIG. 12F is a close up perspective view of the electrode connectiondevice depicted in FIG. 12C, in accordance with at least one embodimentof the present disclosure;

FIG. 12G is a close up plan view of the electrode connection devicedepicted in FIG. 12C, in accordance with at least one embodiment of thepresent disclosure;

FIG. 13A is an exploded view of an illustrative fissile neutrondetection system that uses three neutron moderators and eight neutrondetectors, in accordance with at least one embodiment of the presentdisclosure; and

FIG. 13B is an assembled view of the illustrative fissile neutrondetection system depicted in FIG. 13A, in accordance with at least oneembodiment of the present disclosure.

Although the following Detailed Description will proceed with referencebeing made to illustrative embodiments, many alternatives, modificationsand variations thereof will be apparent to those skilled in the art.

DETAILED DESCRIPTION

A fissile neutron detection system is provided. The fissile neutrondetection system may include a first neutron detector and a secondneutron detector. The first neutron detector and the second neutrondetector may each include: a chamber containing at least one activematerial that emits at least one ionizing particle upon exposure tothermal neutrons and at least one electrode. The system may furtherinclude a neutron moderator disposed proximate the first neutrondetector and the second neutron detector in a space between the firstneutron detector and the second neutron detector. The neutron moderatormay include a hydrogen-containing material that transitions at least aportion of high-energy incident fissile neutrons to low-energy thermalneutrons. In some implementations, the fissile neutron captureefficiency of such a system may exceed 60%. In some implementations, theactive material may include one or more sheets of a solid material suchas lithium-6 (⁶Li) or boron-10 (¹⁰B) that may emit a number of chargedparticle(s) upon capture of a thermal neutron. In some implementations,the neutron moderator may include one or more solid thermoplasticmaterials, such as high-density polyethylene (HDPE), which includes ahigh weight percentage of hydrogen.

A fissile neutron detection method is provided. The method may includetransitioning at least some incident fissile neutrons to thermalneutrons by passing the incident fissile neutrons through at least oneneutron moderator proximate a first neutron detector and a secondneutron detector. The neutron moderator is located in a space betweenthe first neutron detector and the second neutron detector. The neutronmoderator includes a hydrogen containing material that causes thetransition of the at least some incident fissile neutrons to thermalneutron. The method further includes impinging at least a portion of thethermal neutrons exiting the neutron moderator on either of: at leastone active material disposed in a chamber of the first neutron detectoror, at least one active material disposed in a chamber of the secondneutron detector. The method also includes generating, by the firstneutron detector and the second neutron detector, a current proportionalto the number of thermal neutrons impinging on the active material inthe first neutron detector and on the active material in the secondneutron detector.

The relatively high-energy fissile neutron (energy level >100 keV)enters the neutron moderator and collides with hydrogen nuclei withinthe moderator. The collisions reduce the energy level of the fissileneutron to a relatively low-energy thermal neutron (energy level <0.1eV). The collisions may also cause a portion of the incident fissileneutrons to exit the moderator along a vector that does not intersectthe first neutron detector or the second neutron detector. Thus only aportion of the incident fissile neutrons may be captured by the neutrondetectors. Using the detector/moderator/detector arrangement describedherein offers a significant cross-sectional area for fissile neutroncapture, providing capture efficiencies of greater than 60%. Suchcapture efficiency exceeds the capture efficiency of other neutrondetection systems such as systems using ¹⁰B straw detectors placed in amoderator block which offer significantly less cross-sectional area forneutron capture.

At least some of the thermal neutrons exiting the neutron moderatorenter the first neutron detector or the second neutron detector. Withinthe neutron detector, the thermal neutron impinges on an activematerial. In some instances, the active material may capture the thermalneutron and generate a number of daughter particles such as one or morealpha particles (two protons/two neutrons) and/or one or more tritons(one proton/two neutrons). At least some of the daughter particles mayionize a readout gas within the neutron detector. The drift electronsand ionized readout gas cause a charge flow within the neutron detector.The charge flow may be captured by an electrode as an electricalcurrent. One or more properties of the electrical current may beindicative of a number of fissile neutrons detected by the neutrondetector and/or the rate of fissile neutron detection by the neutrondetector.

Another fissile neutron detection system is provided. The fissileneutron detection system may include at least one neutron detector. Eachneutron detector may include a body having a length, a width, and aheight defining a closed chamber; the length and the width of thechamber greater than the height of the chamber, at least one activematerial that emits at least one ionizing particle upon exposure tothermal neutrons, the active material disposed within the chamber and atleast one electrode disposed in the chamber. The fissile neutrondetection system may further include at least one neutron moderatordisposed proximate the at least one neutron detector. Each neutronmoderator may include a material that transitions at least a portion ofhigh-energy incident fissile neutrons to low-energy thermal neutrons.

A fissile neutron detection method is also provided. The method mayinclude transitioning at least some incident fissile neutrons to thermalneutrons by passing the incident fissile neutrons through at least oneneutron moderator disposed proximate at least one neutron detector. Eachneutron detector may include a body having a length, a width, and aheight defining a closed chamber; the length and the width of thechamber greater than the height of the chamber. Each neutron detectormay further include at least one active material that emits at least oneionizing particle upon exposure to thermal neutrons, the active materialdisposed within the chamber and at least one electrode disposed in thechamber. The method may additionally include impinging at least 60% ofthe thermal neutrons exiting the neutron moderator on the at least oneactive material disposed in the chamber of the at least one neutrondetector. The method may further include generating, by the at least oneneutron detector, a current at the at least one electrode, the currentproportional to the number of thermal neutrons impinging on the at leastone active material in the at least one neutron detector.

Another fissile neutron detection system is provided. The fissileneutron detection system may include a first neutron detector, a secondneutron detector, and a neutron moderator. Each of the neutron detectorsmay include a body having a length, a width, and a height that define ahermetically sealed, continuous chamber, wherein the length and thewidth of the body exceed a height of the body. Each neutron detector mayalso include at least one active material disposed within thehermetically sealed chamber, the at least one active material to emit atleast one charged particle upon exposure to a thermal neutron. Eachneutron detector may additionally include at least one electrodedisposed within the chamber and electrically isolated from the body. Theneutron moderator may be disposed proximate the first neutron detectorand the second neutron detector. The neutron moderator may be disposedin a space between the first neutron detector and the second neutrondetector. The neutron moderator may include one or morehydrogen-containing materials that transition at least a portion ofhigh-energy incident fissile neutrons to low-energy thermal neutronsthat are detectable by the first neutron detector or the second neutrondetector.

As used herein, the terms “top” and “bottom” are intended to provide arelative and not an absolute reference to a location. Thus, inverting anobject having a “top cover” and a “bottom cover” may place the “bottomcover” on the top of the object and the “top cover” on the bottom of theobject. Such configurations should be considered as included within thescope of this disclosure.

As used herein, the terms “first,” “second,” and other similar ordinalsare intended to distinguish a number of similar or identical objects andnot to denote a particular or absolute order of the objects. Thus, a“first object” and a “second object” may appear in any order—includingan order in which the second object appears before or prior in space ortime to the first object. Such configurations should be considered asincluded within the scope of this disclosure.

FIG. 1A is a front elevation of an illustrative fissile neutrondetection system 100 that includes a first neutron detector 102A, asecond neutron detector 102B (collectively, “neutron detectors 102”) anda neutron moderator 150 that is at least partially disposed proximatethe first neutron detector 102A and the second neutron detector 102B inthe space formed between the first neutron detector 102A and the secondneutron detector 102B, in accordance with at least one embodiment of thepresent disclosure. FIG. 1B is a partial sectional view of theillustrative neutron detection system 100 shown circled in FIG. 1A, inaccordance with at least one embodiment of the present disclosure. FIG.1B depicts operational level details of the fissile neutron detectionsystem 100. FIG. 1C is a side elevation of the illustrative fissileneutron detection system 100 depicted in FIG. 1A, in accordance with atleast one embodiment of the present disclosure. FIG. 1D is a partialsectional view of the illustrative neutron detection system 100 showncircled in FIG. 1C, in accordance with at least one embodiment of thepresent disclosure. FIG. 1D depicts operational level details of thefissile neutron detection system 100. FIG. 1E is a partial sectionalview of an illustrative isolator 128 through which an electrode 116 isdisposed within the neutron detector 102.

Although a first neutron detector 102A and a second neutron detector102B are depicted and described in association with FIGS. 1A-1E,alternate arrangements that include only a single neutron detector 102should be considered as alternate embodiments of the concepts describedherein. For example, a single neutron detector 102 may be formed into ahollow ring or tube-like structure having one or more aperturestherethrough. In such an implementation, the neutron moderator 150 maybe disposed in whole or in part within some or all of the apertures.

The neutron moderator 150 may be a single or multi-piece member havingany number or combination of shapes or configurations. Regardless ofshape and/or configuration, the neutron moderator 150 has a surface areathat may be defined by the external surfaces of the neutron moderator150. Such surface area may include exposed (e.g., outwardly facing)exterior surfaces, hidden (e.g., inwardly facing) exterior surfaces, orany combination thereof. For example, the surface area of a planarneutron modulator 150 having an annulus therethrough would include thesurface area of the plane, the “edges” of the plane, and the surfacearea about the perimeter of the annulus. A majority of the surface areaof the neutron moderator 150 may be disposed proximate one or moreneutron detectors 102. In various embodiments, the majority of thesurface area of the neutron moderator 150 may include a surface areathat is: greater than about 50% of the total moderator surface area;greater than about 60% of the total moderator surface area; greater thanabout 70% of the total moderator surface area; greater than about 80% ofthe total moderator surface area; greater than about 90% of the totalmoderator surface area; greater than about 95% of the total moderatorsurface area; or greater than about 99% of the total moderator surfacearea.

At least a portion of the fissile neutrons 160 impinging upon theneutron moderator 150 may enter, strike, or otherwise impinge upon theneutron moderator 150. Within the neutron moderator 150 energy may bestripped as the fissile neutrons collide with hydrogen nuclei within themoderator. As a result of these collisions, the energy level of thefissile neutron 160 is reduced to that of a thermal neutron. Also as aresult of the random nature of these collisions, the low-energy thermalneutron 162 may exit the neutron moderator along the same or differentvector than the incident fissile neutron 160. By placing the majority ofthe surface area of the neutron moderator 150 proximate a neutrondetector 150, the probability of detecting such a thermal neutron 162exiting the neutron moderator is increased. In various embodiments, theprobability that an incident fissile neutron 160 will be detected uponexiting the neutron moderator 150 as a thermal neutron 162 may be: about60% or greater; about 70% or greater; about 80% or greater; about 90% orgreater; about 95% or greater; about 99% or greater.

Each of the neutron detectors 102 includes a top cover 104, a bottomcover 108, and sidewalls 120 that form a chamber 105. In someimplementations, one or more gas tight or gas impervious seals 124 maybe disposed in the joints formed by the sidewall 120 and the top cover104 and the sidewall 120 and the bottom cover 108. In at least someimplementations, the seals 124 may isolate or hermetically seal thechamber 105 to minimize or even prevent exchange of gases or fluidsbetween the chamber 105 and the exterior environment. In someimplementations, the chamber 105 may include a single, continuous (i.e.,uninterrupted) chamber 105. In some implementations the chamber 105 maycontain one or more gases or gas combinations. For example, the chamber105 may contain a noble gas such as argon (Ar). In some implementations,at least one layer or sheet of active material 112, a support matrix 106to support each layer or sheet of active material 112. A number ofelectrodes 116 may extend partially or completely through all or aportion of the chamber 105. Isolators 128 may be disposed at locationswhere the electrodes 116 extend through a wall or cover of the chamber105 to electrically isolate the electrode 116 from the sidewall or coverof the chamber 105. Although not depicted in FIGS. 1A-1E, in someimplementations, all or a portion of the number of electrodes 116 maypenetrate the top cover 104 and/or bottom cover 108 to enter the chamber105 of the neutron detector 102 rather than penetrating the sidewall120. In such instances, one or more isolators 128 may be disposed aboutthe electrodes 116 at the point the electrodes penetrate into thechamber 105. As depicted in FIGS. 1B and 1D, during operation, neutronsand gamma rays impinge upon the fissile neutron detection system 100.Fissile neutrons 160 may be produced, for example, by plutonium (Pu) orother highly enriched uranium (HEU) products such as may be found in a“dirty bomb” or other explosive device. Within the chamber 105 of eachof the neutron detectors 102 a sheet of active material 112, such aslithium 6 (⁶Li) may be disposed on a support structure such as analuminum honeycomb matrix 106. Fissile neutrons impinging upon thefissile neutron detection system 100 may pass through a neutronmoderator 150 where the energy level of at least a portion of theincident fissile neutrons 160 (e.g., 100 keV to 200 MeV) may be reducedto the energy level of a thermal neutron 162 (e.g., 0.025 electron volts(eV) to 0.50 electron volts (eV)).

The thermal neutron 162 may be captured by one of the ⁶Li atomscontained in the active material 112. The capture of the thermal neutron162 by the ⁶Li atom forms a lithium 7 (⁷Li) atom that may decay into twodaughter particles, an alpha particle 166 and a triton 168. The triton168 and alpha particle 166 travel in opposite directions, and dissipateenergy as they travel through the active material 112. Upon exiting theactive material 112, at least some of the tritons 168 or alpha particles166 having sufficient kinetic energy will ionize atoms in the readoutgas 170 disposed within the chamber 105. Electrons 172 produced by theionization of the readout gas 170 may tend to drift towards theelectrodes 116 in the chamber and the ionized gas generated by theionization of readout gas 170 may tend to drift towards the active layer112. Electrons 172 that drift within the amplification region 176 (i.e.,the Townsend avalanche region—approximately 5 times the radius of theelectrode 116) encounter an electric field that accelerates the driftingelectrons 172 to a sufficient velocity that additional readout gas 170may be ionized. The additional ionized readout gas may create additionalelectrons 172 that also tend to drift toward the electrodes 116 andcause additional ionization of the readout gas 170. This process thatoccurs within the Townsend avalanche may be referred to as “gasmultiplication.” Ionized atoms of the readout gas 170 within theTownsend avalanche region that move towards the active layer 112 inducea current flow along the electrode 116. In implementations, the currentalong the electrode 116 may be collected and amplified using apulse-mode, charge-sensitive preamplifier to produce a voltage outputsignal 192. Pulse height discrimination circuitry may be used to comparethe voltage output signal to a first defined threshold to determinewhether a fissile neutron 160 has been detected (e.g., for a gasmultiplication of roughly 100, and an amplification circuitry gain ofabout 1 fC/mV, pulse heights greater than about 250 keV may indicate thepresence and/or detection of a fissile neutron 160).

In some embodiments, the false positive detection rate of fissileneutrons 160 based on the first predetermined threshold may be less than1×10⁻⁵ for a gamma ray exposure rate of 100 mR/hr. A secondpredetermined threshold may be selected and may be set at a value thatis less than the first predetermined threshold. Voltage output signals192 below the second predetermined threshold may be deemed as very lowionizing gamma ray events or movements of charge in the fissile neutrondetection system 100 that were induced by another source (e.g., thermalheat, radio frequency electromagnetic radiation, and changes in therelative position of the electrodes 116 and active layer 112—known asmicrophonics). Voltage output signals 192 below the first predeterminedthreshold and above the second predetermined threshold may be indicativeof gamma ray events. The detected rate of neutrons and gamma raysimpinging upon the fissile neutron detection system 100 can be used inradiation detection methodologies (e.g., detect the presence of anuclear weapon or unauthorized nuclear device).

In embodiments, the composition of the readout gas 170 may be maintainedrelatively constant over time to avoid deterioration of the gamma rayand neutron detection process. Change in readout gas 170 compositiongreater than 1% in the composition may affect the Townsend avalancheprocess. For example, nitrogen, oxygen, or water molecules that leakinto the chamber 105 may not ionize as well as a readout gas 170, suchas argon, in the amplification region 176 near the electrodes 116, andtherefore may reduce the Townsend avalanche process near the electrodes116 when introduced into the readout gas 170. This may reduce theability of the readout electronics to distinguish between noise, gammaray, and fissile neutron events, thereby decreasing the efficiencyand/or accuracy of the fissile neutron detection system 100. A 1% changein the composition of the readout gas 170 may cause up to an 8% changein the voltage output signal 192. To maintain accuracy andresponsiveness of the fissile neutron detection system, it isadvantageous to limit the change in composition of the readout gas 170by minimizing the following: (1) the egress of readout gas 170 from thechamber 105; and (2) the ingress of contaminants, including airconstituents (nitrogen, oxygen, carbon dioxide), water, and otherairborne molecules, into the chamber 105.

The top cover 104 and the bottom cover 108 may be fabricated from one ormore materials that permit the passage of thermal neutrons 162 from aneutron moderator 150 to the chamber 105. In at least someimplementation, the top cover 104 and the bottom cover 108 may befabricated from one or more stainless steels, such as, an 18/8 stainlesssteel, a 304 stainless steel, a 304L stainless steel, a 316 stainlesssteel, or a 316L stainless steel. Other grades and materials may besubstituted with equal efficiency. The top cover 104 and the bottomcover 108 define the overall configuration of the neutron detector 102.In one example, the top cover 104 and the bottom cover 108 may includegenerally planar members—in such embodiments, the neutron detector 102may have a generally planar configuration, for example a squareconfiguration having a side length of about 0.10 meters (m) or less;about 0.25 m or less, about 0.50 m or less, about 0.75 m or less, about1 m or less, or about 2 m or less. The top cover 104 and the bottomcover 108 may have other shapes, such as triangular, octagonal,hexagonal, circular, elliptical, rectangular, or even irregular shapesto fit within designated areas. Similarly, the chamber 105 at leastpartially formed and/or bounded by the top cover 104 and/or the bottomcover 108 may have any shape, configuration, or regular/irregularperimeter. For example, the chamber 105 may be generally square,generally rectangular, generally oval, generally elliptical, generallycircular, generally triangular, generally polygonal, generallytrapezoidal, or any other regular or irregular configuration. In someimplementations, all or a portion of the chamber 105 may be spherical orhemispherical and all or a portion of the neutron moderator 150 may bespherical and placed concentrically within the chamber 105 of theneutron detector 102.

The chamber 105 may have any dimensions. In some embodiments, thechamber 105 may be defined by three, mutually orthogonal, measurements,such as a length, a width, and a height. In such embodiments, the topcover 104 and/or the bottom cover 108 may define either or both thelength and the width of the chamber 105. In such embodiments, thesidewall 120 may define the height of the chamber 105. The chamber 105may have a length and width that greatly exceed the height of thechamber 105. In some embodiments, the length of the chamber 105 measuredalong a first axis may exceed the height of the chamber 105 measuredalong a second axis orthogonal to the first axis by a factor of: about 3times or greater; about 5 times or greater; about 7 times or greater;about 10 times or greater; about 15 times or greater; about 20 times orgreater; about 25 times or greater; about 30 times or greater; about 50times or greater; about 75 times or greater; or about 100 times orgreater. In some embodiments, the width of the chamber 105 measuredalong a third axis may exceed the height of the chamber 105 measuredalong the second axis orthogonal to the third axis by a factor of: about3 times or greater; about 5 times or greater; about 7 times or greater;about 10 times or greater; about 15 times or greater; about 20 times orgreater; about 25 times or greater; about 30 times or greater; about 50times or greater; about 75 times or greater; or about 100 times orgreater.

In embodiments, the chamber 105 may have a length, measured along afirst axis, of about 10 centimeters (cm) or greater; about 20 cm orgreater; about 30 cm or greater; about 50 cm or greater; about 75 cm orgreater; about 100 cm or greater; about 200 cm or greater; about 500 cmor greater; about 700 cm or greater; or about 1000 cm or greater. Inembodiments, the chamber 105 may have a height, measured along a secondaxis orthogonal to the first axis, of about 0.5 centimeters (cm) orless; about 1 cm or less; about 1.5 cm or less; about 2 cm or less;about 2.5 cm or less; about 3 cm or less; about 3.5 cm or less; about 4cm or less; about 4.5 cm or less; or about 5 cm or less. In embodiments,the chamber 105 may have a width, measured along a third axis orthogonalto the first axis and the second axis, of about 10 centimeters (cm) orless; about 15 cm or less; about 20 cm or less; about 25 cm or less;about 30 cm or less; about 35 cm or less; about 40 cm or less; about 45cm or less; about 50 cm or less; or about 100 cm or less.

In other embodiments, the top cover 104 and the bottom cover 108 mayhave configurations other than planar, for example the top cover 104 mayinclude a simple or compound curved surface having a first radius whilethe bottom cover 108 may include a similar simple or compound curvedsurface having a second radius that is greater or less than the firstradius. Such an implementation can provide a neutron detector that iscurved, arced, or hemispherical.

In yet other embodiments, the top cover 104 and the bottom cover 108 mayhave generally similar irregular shapes that permit the construction ofneutron detectors 102 having virtually any size, shape, and/or physicalconfiguration. Such may, for example, advantageously permit the customfitting of neutron detectors 102 within odd or irregular shapedhousings. In at least some implementations, all or a portion of the topcover 104 and/or the bottom cover 108 may be integrally formed with allor a portion of the sidewall 120 to eliminate one or more joints betweenthe respective cover 104, 108 and the sidewall 120. In someimplementations, all or a portion of the top cover 104 and/or bottomcover 108 may be affixed to all or a portion of the sidewall 120 usingone or more adhesives, by welding or brazing, or similar attachment oraffixment techniques capable of providing a gas tight seal between thesidewall 120, the top cover 104, and the bottom cover 108. In someimplementations, the top cover 104, the bottom cover 108, and at least aportion of the sidewall 120 may be integrally formed, for example usingone or more casting, extrusion, injection molding, or similar processesin which all or a portion of the top cover 104, all or a portion of thebottom cover 108, and a portion of the sidewalls 120 are seamlesslyformed.

In some embodiments, some or all of the seals 124 between the sidewall120 and the top cover 104 and/or the sidewall 120 and the bottom cover108 may be formed from an elastomeric compound that is compressed orotherwise formed to the mating surfaces of the sidewall 120 and the topcover 104 and/or the sidewall 120 and the bottom cover 108. In someimplementations all or a portion of the seals 124 may includepolyisobutylene or one or more polyisobutylene containing compounds tomaintain the composition of the readout gas 170 over an extendedtimeframe (e.g., 30 years). Beneficially, the use of a flexibleelastomeric seal 124 provides the ability for the seal 124 to conform tothe mating surfaces of the sidewall 120, the top cover 104 and/or thebottom cover 108, filling any imperfections in the mating surfaces andminimizing the likelihood of readout gas 170 leakage through gaps formedby imperfections in the mating surfaces.

In some embodiments, the quality of the mating surfaces found on the topcover 104, bottom cover 108, and/or sidewalls 120 may be selected togenerate uniform electric fields near the electrodes 116 of the fissileneutron detection system 100 (e.g., the variance in the finish on themating surfaces of the top cover 104, bottom cover 108, and/or sidewall120 may be equal to or less than 0.020″ inches). Providing such asurface finish on the mating surfaces improves sealing of the chamber105 and takes advantage of the sealing properties of an elastomeric seal124 which may accommodate such fluctuations in the surface finish of themating surfaces.

The use of an elastomeric seal 124 may also facilitate a low temperaturemanufacturing process that minimizes or even eliminates high temperatureprocesses, such as welding or brazing, on the fissile neutron detectionsystem 100 which reduces warping and bending of the components of thefissile neutron detection system 100. An elastomeric seal may alsoaccommodate thermal expansion/contraction of the chamber components,thereby allowing a greater number of material choices for the top plate104, bottom plate 108, and sidewalls 120 such as glass, aluminum, orstainless steel. The elastomeric seal 124 may have a thickness in therange of about 25 micrometers (μm) to about 1 centimeter (cm) and awidth in the range of about 1 cm to about 5 cm. Such an elastomeric seal124 may provide less than 1% leakage of an argon-methane readout gas 170from the chamber 105, and less than 1% leakage of oxygen into thechamber 105 over a 30 year period for a chamber 105 having a length ofabout 0.5 m, a width of about 1 m, and a height of about 1 cm. In oneimplementation, the elastomeric seal 124 may include a polyisobutyleneseal 124 having a width of about 1.5 cm, a total surface area of 30square centimeters (cm²), may maintain an oxygen leak rate into thechamber 105 of about 1.3×10⁻¹⁰ cm³·cm/(s·cm²·cm-Hg). A leak rate ofabout 1.3×10⁻¹⁰ cm³·cm/(s·cm²·cm-Hg) provides an oxygen concentration ofabout 0.75% by volume for a chamber 105 having a volume of approximately5000 cubic centimeter (cm³) after 30 years of operation.

In some embodiments, all or a portion of the sidewalls 120 may befabricated using one or more metallic materials, such as stainlesssteel. In some embodiments, all or a portion of the sidewall 120 may befabricated using aluminum or an aluminum containing alloy. In someembodiments, the sidewall 120 may have a mating surface or lip that,upon assembly, is disposed proximate the top cover 104, the bottom cover108, or both the top cover 104 and the bottom cover 108. In someimplementations, the mating surface may be machined or similarlyfinished to remove irregularities in the surface and provide arelatively smooth sealing surface.

In some embodiments, the readout gas 170 may include one or more pure ornearly pure noble gases, such as argon (Ar). In some embodiments, thereadout gas 170 may include a gas mixture, for example a gas containing90 percent by volume (vol %) argon and 10 vol % quenching gas such ascarbon dioxide or methane. In some implementations, the voltage biasapplied to the electrode 116 may be adjusted, controlled, or otherwisealtered based at least in part on the composition of the readout gas170. In such instances, a small (e.g., 1%) change in the bias voltageapplied to an electrode 116 may cause a larger change (e.g., up to about15%) in the voltage output signal provided by or generated by theelectrode 116.

The active material 112 disposed in the chamber 105 may include one ormore sheets of active material disposed within the chamber, one or morelayers of active material disposed within the chamber, or in someimplementations, an active gas disposed within the chamber 105. In someimplementations, the active material may include lithium 6 (⁶Li), boron10 (¹⁰B), or helium 3 (³He). In some embodiments, the active material112 may include a sheet of active material such as a sheet of ⁶Li foilthat, in some embodiments, may be supported within the chamber 105 by asupport matrix 106. In such implementations, the length and width of thesheet of active material 112 may greatly exceed the thickness of thelayer of active material 112. In such implementations, the length andwidth of the sheet of active material 112 may greatly exceed thethickness of the neutron detector 102. In such implementations, thelength and width of the sheet of active material 112 may greatly exceedthe thickness of the chamber 105.

In embodiments using one or more sheets of ⁶Li foil as the activematerial 112, each sheet of ⁶Li foil may have a length and a with thatgreatly exceed the thickness of the foil. In embodiments, the sheet of⁶Li foil may have a thickness of about 30 micrometers (μm) or less;about 40 μm or less; about 50 μm or less; about 60 μm or less; about 70μm or less; about 80 μm or less; about 90 μm or less; about 100 μm orless; about 110 μm or less; or about 120 μm or less. In embodiments, thesheet of ⁶Li foil may have a width of about 10 centimeters (cm) or less;about 20 cm or less; about 30 cm or less; about 40 cm or less; about 50cm or less; about 60 cm or less; about 70 cm or less; about 80 cm orless; about 90 cm or less; about 100 cm or less. In embodiments, thesheet of ⁶Li foil may have a length of about 10 centimeters (cm) orless; about 25 cm or less; about 50 cm or less; about 75 cm or less;about 100 cm or less; about 150 cm or less; about 200 cm or less; about250 cm or less; about 500 cm or less; about 1000 cm or less.

Embodiments in which the active layer 112 is disposed at an intermediatepoint within the chamber may advantageously detect tritons 166 emittedfrom both sides of the active layer 112. In contrast, tritons 166emitted from only one side of the active layer 112 may be detected inembodiments in which the active layer 112 is disposed proximate the topcover 104 and/or bottom cover 108 rather than disposed at anintermediate point in the chamber 105.

In some embodiments, the active material 112 may include a layer ofactive material such as a layer of ¹⁰B that may be disposed on substratethat is disposed within the chamber 105. In some embodiments, the activematerial 112 may include a layer of active material such as a layer of¹⁰B that may be disposed (e.g., via chemical vapor deposition or similarprocesses) on all or a portion of an interior surface of the top cover104, bottom cover 108, and/or sidewalls 120 forming the chamber 105. Insuch implementations, the length and width of the layer of activematerial 112 may greatly exceed the thickness of the layer of activematerial 112. In such implementations, the length and width of the layerof active material 112 may greatly exceed the thickness of the neutrondetector 102. In such implementations, the length and width of the layerof active material 112 may greatly exceed the thickness of the chamber105.

In some implementations, the active material 112 may include one or moreactive gas species, for example helium 3 (³He). In such instances, thechamber 105 may be filled with one or more active gases or a mixturethat includes one or more active gases. In some implementations, acombination of active sheets, active layers, and/or active gases may bedisposed within the chamber 105.

In some implementations, all or a portion of the top cover 104 and/orthe bottom cover 108 may be formed into a dished or tray-like form suchthat the top cover 104 and/or the bottom cover 108 form at least aportion of the sidewall 120, and may, on occasion, form the entiresidewall 120 of the chamber 105. In some implementations, the neutrondetector 102 may have a thickness (that includes the top cover 104, thesidewall 120 (if present), and the bottom cover 108 of: about 0.5centimeters (cm) or less; about 1 cm or less; about 1.5 cm or less;about 2.0 cm or less; about 2.5 cm or less; about 3.0 cm or less; about3.5 cm or less; about 4.0 cm or less; about 4.5 cm or less; about 5.0 cmor less.

The top cover 104 and the bottom cover 108 may have any dimensions,geometry, and/or configuration to provide a neutron detector 102 havingany shape or geometry. In some implementations, the neutron detector 102may be in the physical configuration of a planar structure having alength and width that greatly exceeds the thickness of the detector 102.In some implementations the length of the neutron detector 120, measuredalong a first axis, may be from about 5 or more times the thickness ofthe detector 102 to about 100 or more times the thickness of thedetector 102. In some implementations the width of the neutron detector102, measured along a second axis that is orthogonal to the first axis,may be from about 3 or more times the thickness of the detector 102 toabout 50 or more times the thickness of the detector 102. In someimplementations, the neutron detector 102 may have a length, measuredalong a first axis, of from about 10 centimeters (cm) or greater toabout 1000 cm or greater; a thickness, measured along a second axisorthogonal to the first axis, of from about 0.5 centimeters (cm) or lessto about 5 cm or less; and a width, measured along a third axisorthogonal to the first axis and the second axis of from about 30 cm toabout 500 cm. In such implementations, the top cover 104 and the bottomcover 108 may have a corresponding width of from about 30 cm to about500 cm; and a corresponding length of from about 10 cm or less to about100 cm or less.

Other neutron detector 102 physical configurations are possible. Forexample, the neutron detector 102 may be curved about a single axis toprovide a neutron detector 102 having a chamber 105 that is arced orparabolic. In such an implementation, the top cover 104 and the bottomcover 108 may be arced or parabolic along the desired axis to providethe chamber 105. In another example, the neutron detected 102 may becurved about two axes to provide a neutron detector 102 having a chamber105 that is a concave dish, a convex dish, or hemispherical. In such animplementation, the top cover 104 and the bottom cover 108 may be arcedor dished along the respective axes to provide the arced or dishedchamber 105. In some implementations, the top cover 104 and/or thebottom cover 108 may be fabricated using one or more stainless steels,aluminum, or one or more aluminum alloys. The top cover 104 and/or thebottom cover 108 can be made of glass such as soda-lime or borosilicateglass.

In some embodiments, some or all of the electrodes 116 may pass throughthe sidewall 120 of the neutron detector 102. In some embodiments, someor all of the electrodes 116 may pass through the top cover 104 and/orthe bottom cover 108 of the neutron detector 102. Any number ofelectrodes 116 may be disposed within the chamber 105. Each of theelectrodes 116 can have any profile or shape, for example, theelectrodes 116 may include conductors having a round cross section witha diameter of from about 25 micrometers (μm) to about 150 μm. Inembodiments, the electrodes 116 may be tensioned to about 33% to about67% of the breaking or failure limit for the electrode material.

One or more isolators 128 may electrically isolate the electrodes 116from the sidewall 120, top cover 104, and/or bottom cover 108 of theneutron detector 102. In some implementations, the one or more isolators128 may hermetically seal about the electrode 116, thereby maintainingthe hermetic integrity of the chamber 105. In some implementations, eachof the one or more isolators 128 may permit the passage of an electrode116 through an aperture extending through the isolator 128. Afterpassing the electrode 116 through the isolator 128, the space around theisolator 128 may be filled using a material such as solder, conductiveepoxy, brazing, or welding. The tube length through the isolator 128 andthe inner diameter of the isolator 128 may be selected based on avariety of factors. For example, the shear strength of Sn-37Pb andSn-3.5Ag solder may exceed 3000 pounds per square inch (psi). With atension of approximately 450 grams or 1 pound on a 50 μm diametertungsten rhenium wire, a solder length of approximately 7 millimeters(mm) would provide a safety factor of 5. The isolators 128 may includeany current or future developed electrical insulator. Non-limitingexamples of such electrical insulators may include, but are not limitedto, glass isolators, ceramic isolators, Bakelite isolators, resinisolators, epoxy isolators, and similar.

In some implementations, the neutron detector 102 may include one ormore isolator feedthrough inserts 126. Beneficially, the one or moreisolator feedthrough inserts 126 may be manufactured separate from theneutron detector 102 using a separate process that provides aglass-to-metal or ceramic-to-metal feedthrough assembly process. Suchconstruction permits the formation of a hermetic seal between the one ormore isolator feedthrough inserts 126, the isolator 128 and theelectrode 116 without requiring the one or more isolator feedthroughinserts 126 be incorporated during the manufacturing process of theneutron detector 102. The one or more isolator feedthrough inserts 126may be modularly constructed and may contain any number of electrodes116. The one or more isolator feedthrough inserts 126 may be affixed tothe neutron detector 102 via one or more processes such as welding orbrazing.

In some implementations, the electrodes 116 may be disposed generallyparallel to each other and extending from a first side of the neutrondetector 102 to a second side of the neutron detector 102. Otherelectrode configurations are possible, for example, implementations inwhich some or all of the electrodes 116 are arranged in a pattern suchas a star pattern in which the electrodes 116 are not parallel to eachother. In various embodiments, the electrodes 116 may be maintained atthe same potential or different potentials. For example, in neutrondetectors 102 using a sheet type active material 112, an electricalsource 190 may maintain the electrodes 116 at a positive or negativepotential measured with respect to the sheet-type active material 112.In some implementations, the electrodes 116 may be maintained at apotential of about +25 volts (V) greater than the active material 112;about +50 V greater than the active material 112; about +75 V greaterthan the active material 112; about +100 V greater than the activematerial 112; about +125 V greater than the active material 112; about+150 V greater than the active material 112; or about +200 V greaterthan the active material 112.

The moderator 150 includes one or more materials capable of reducing anenergy level of a fissile neutron 160 to an energy level of a thermalneutron 162. Such reduction in energy level of the fissile neutron 160occurs within the moderator 150 as the fissile neutron 160 impactsnuclei in the moderator 150. The moderator 150 may include one or morematerials that include a minimum of about 30 weight percent hydrogen;about 35 weight percent hydrogen; about 40 weight percent hydrogen;about 45 weight percent hydrogen; about 50 weight percent hydrogen; orabout 55 weight percent hydrogen. The moderator 150 may include one ormore solids, one or more liquids, and/or one or more compressed gases,or combinations thereof. The use of moderators containing predominantlylarger nuclei (e.g., carbon) may disadvantageously cause ricocheting(rather than the preferred slowing) of the incident fissile neutrons160.

In at least some implementations, all or a portion of the moderator 150may be disposed between a first neutron detector 102A and a secondneutron detector 102B. In some implementations, no air gap or similarvoid may exist between the moderator 150 and the exterior surface of thetop cover 104 and/or the exterior surface of the bottom cover 108 of theneutron detector 102. In some implementations, an air gap or similarvoid space may exist between the moderator 150 and the exterior surfaceof the top cover 104 and/or the exterior surface of the bottom cover 108of the neutron detector 102. In some implementations, one or morehydrogenated gels or similar materials that improve the transport ofthermal neutrons 162 from the moderator 150 to the neutron detector maybe disposed between the moderator 150 and the exterior surface of thetop cover 104 and/or the exterior surface of the bottom cover 108 of theneutron detector 102. In some implementations, one or more gel packs orsimilar devices containing one or more solid, semisolid, or liquidhydrogenated materials, fluids, gels, or liquids, may be disposedbetween the first neutron detector 102A and the second neutron detector102B. In at least one implementation, the moderator 150 may include asolid thermoplastic material such as high-density polyethylene (HDPE).

In some implementations, the thickness of the moderator 150 disposedbetween the first neutron detector 102A and the second neutron detector102B may have a constant thickness that is greater than the thickness ofeither the first neutron detector 102A and/or the second neutrondetector 102B. In some implementations, the moderator 150 may have alength and a width that is about the same as the length and the width ofthe first neutron detector 102A and the second neutron detector 102B. Inembodiments, the moderator 150 may have a length that is about 50centimeters (cm) or more; about 75 cm or more; about 100 cm or more;about 125 cm or more; about 150 cm or more; about 200 cm or more; about300 cm or more; about 400 cm or more; or about 500 cm or more. Inembodiments, the moderator 150 may have a width that is about 50centimeters (cm) or more; about 75 cm or more; about 100 cm or more;about 125 cm or more; about 150 cm or more; about 200 cm or more; about300 cm or more; about 400 cm or more; or about 500 cm or more. Inembodiments, the moderator 150 may have a thickness that is about 5centimeters (cm) or less; about 4 cm or less; about 3 cm or less; about2 cm or less; or about 1 cm or less. In some implementations, thethickness of the moderator 150 may be based in whole or in part on thethickness of either or both neutron detectors 102 adjacent to themoderator 150. In embodiments, the thickness of the moderator 150 may beabout 1.0 to about 1.25 times the thickness of the adjacent neutrondetector 102; about 1.0 to about 1.5 times the thickness of the adjacentneutron detector 102; about 1.0 to about 1.75 times the thickness of theadjacent neutron detector 102; about 1.0 to about 2.0 times thethickness of the adjacent neutron detector 102; or about 1.0 to about5.0 times the thickness of the adjacent neutron detector 102. In someimplementations, the moderator 150 may include one or more materialshaving a length and width that both greatly exceed the thickness of themoderator 150.

FIG. 2A is a cross-sectional view of an illustrative fissile neutrondetector 200 that includes a first neutron detector 102A, a secondneutron detector 102B, and a neutron moderator 150 disposed proximatethe first neutron detector 102A and the second neutron detector 102B atleast partially within a space formed between the first neutron detector102A and the second neutron detector 102B, in accordance with at leastone embodiment of the present disclosure. FIG. 2B is a cross-sectionalview of another illustrative fissile neutron detector 200 that includesa first neutron detector 102A, a second neutron detector 102B, and athird neutron detector 102C, in accordance with at least one embodimentof the present disclosure. FIG. 2B depicts a first neutron moderator150A disposed proximate the first neutron detector 102A and the secondneutron detector 102B at least partially within a space formed betweenthe first neutron detector 102A and the second neutron detector 102B anda second neutron moderator 150B disposed proximate the second neutrondetector 102B and the third neutron detector 102C at least partiallywithin a space formed between the second neutron detector 102B and thethird neutron detector 102C.

As depicted in FIGS. 2A and 2B, the neutron detectors 102 and theneutron moderator(s) 150 may be at least partially enclosed by anexternal neutron moderator 202. In some implementations a housing orshell 204 may be disposed about all or a portion of the external neutronmoderator 202. The thickness of the external moderator 202 may be thesame or different in various areas of the neutron detectors 102. Forexample, the external neutron moderator 202 may have a first thickness212 proximate at least a portion of the first neutron detector 102A(e.g., on the “top” or exposed portion of the fissile neutron detectionsystem 200) and a second thickness 214 proximate at least a portion ofthe second neutron detector 102B (FIG. 2A) or third neutron detector102C (FIG. 2B)—e.g., on the “bottom” or the portion of the fissileneutron detection system 200. In addition, the external neutronmoderator 202 may have a third thickness 216 proximate the sides of theneutron detectors 102.

The external moderator 202 includes one or more materials capable ofreducing an energy level of an incident fissile neutron 160. Suchreduction in energy level of the incident fissile neutron 160 occurswithin the external moderator 202 as the fissile neutron 160 impactshydrogen nuclei in the material forming the external moderator 202. Theexternal moderator 202 may include one or more materials that include aminimum of about 30 weight percent hydrogen; about 35 weight percenthydrogen; about 40 weight percent hydrogen; about 45 weight percenthydrogen; about 50 weight percent hydrogen; or about 55 weight percenthydrogen. The external moderator 202 may include one or more solids, oneor more liquids, and/or one or more compressed gases, or combinationsthereof. In some instances, the external moderator 202 may include oneor more materials, such as one or more heavier molecular weightcompounds, that cause the incident fissile neutrons 160 to enter orfocus the incident fissile neutrons 160 on the neutron detectors 102included in the fissile neutron detection system 200. In at least someimplementations, the external moderator 202 may partially or completelyinclude a hydrogen-containing, solid, thermoplastic, material such ashigh-density polyethylene (HDPE).

In implementations, the first thickness 212, the second thickness 214,and the third thickness 216 of the external moderator 202 may be thesame or different. The external moderator 202 may have a first thickness212 of about 1 centimeter (cm) or less; about 2 cm or less; about 3 cmor less; about 5 cm or less; about 7 cm or less; or about 10 cm or less.The external moderator 202 may have a second thickness 214 of about 1centimeter (cm) or less; about 2 cm or less; about 3 cm or less; about 5cm or less; about 7 cm or less; or about 10 cm or less. The externalmoderator 202 may have a third thickness 216 of about 1 centimeter (cm)or less; about 2 cm or less; about 3 cm or less; about 5 cm or less;about 7 cm or less; or about 10 cm or less.

In at least some implementations, all or a portion of the externalmoderator 202 may be disposed proximate the neutron detectors 102forming the fissile neutron detection system 200. In someimplementations, no air gap or similar void may exist between theexternal moderator 202 and the exterior surface of the neutron detectors102 forming the fissile neutron detection system 200. In someimplementations, an air gap or similar void space may exist between theexternal moderator 202 and the exterior surface of the neutron detectors102. In some implementations, one or more hydrogenated gels or similarmaterials that improve the transport of fissile neutrons 160 and/orthermal neutrons 162 from the external moderator 202 to the neutrondetectors 102 in the fissile neutron detection system 200 may bedisposed between the external moderator 202 and the exterior surfaces ofthe neutron detectors 102.

As depicted in FIGS. 2A and 2B, each of the neutron detectors 102 isseparated by a neutron moderator 150 having a thickness 210. In someimplementations, each of the neutron detectors 102 may be separated by aneutron moderator 150 having a constant thickness 210. In someimplementations, each of the neutron detectors 102 may be separated by aneutron moderator 150 having a variable thickness 210. In someimplementations that include a plurality of neutron moderators 150A-150n (e.g., FIG. 2B), each of the neutron moderators 150 may have the sameor a different constant thickness 210. In some implementations thatinclude a plurality of neutron moderators 150A-150 n (e.g., FIG. 2B),each of the neutron moderators 150 may have the same or a differentvariable thickness 210.

FIG. 3 depicts a hypothetical performance 300 of an illustrative fissileneutron detection system 200 such as depicted and described in detailwith regard to FIG. 2A in which a first neutron detector 102A and asecond neutron detector 102B are separated by a neutron moderator 150,in accordance with at least one embodiment of the present disclosure. Asdepicted in FIG. 3, each of the neutron detectors 102 may include astainless steel, hermetically sealed, chamber 105 that may have athickness (i.e., a sidewall 120 height) of from about 1.5 centimeters(cm) to about 2.5 cm. Simulations were performed for various ⁶Li activematerial 112 sheet thicknesses and neutron moderator 150 thicknesses.Chart 310 shows the capture performance of the two neutron detectorfissile neutron detection system 200 depicted in FIG. 2A as a density orcolor plot of ⁶Li active material sheet thickness 312 along the x-axisversus neutron moderator thickness 314 along the y-axis. In thisembodiment, capture performance peaks in the region of 320, at a ⁶Liactive material sheet thickness of from about 80 micrometers (μm) toabout 100 μm in combination with a neutron moderator thickness of fromabout 3 centimeters (cm) to about 4 cm.

FIG. 4 depicts a hypothetical performance 400 of an illustrative fissileneutron detection system 200 such as depicted and described in detailwith regard to FIG. 2B in which a first neutron detector 102A, a secondneutron detector 102B, and a third neutron detector 102C are separatedby respective neutron moderators 150A and 150B, in accordance with atleast one embodiment of the present disclosure. As depicted in FIG. 4,each of the neutron detectors 102 may include a stainless steel,hermetically sealed, chamber 105 that may have a thickness (i.e., asidewall 120 height) of from about 1.5 centimeters (cm) to about 2.5 cm.Simulations were performed for various ⁶Li active material 112 sheetthicknesses and neutron moderator 150 thicknesses. Chart 410 shows thecapture performance of the two neutron detector fissile neutrondetection system 200 depicted in FIG. 2B as a density or color plot of⁶Li active material sheet thickness 412 along the x-axis versus neutronmoderator thickness 414 along the y-axis. In this embodiment, captureperformance peaks in the region of 420, at a ⁶Li active material sheetthickness of from about 80 micrometers (μm) to about 100 μm incombination with a neutron moderator thickness of from about 1.5centimeters (cm) to about 2.5 cm.

FIG. 5 is an illustrative chart 500 depicting the comparativeperformance of a first neutron detector 102A that includes a first ⁶Liactive layer 112A disposed on an interior surface of the top cover 104and a second ⁶Li active layer 112B disposed on an interior surface ofthe bottom cover 104 of the neutron detector 102A against a secondneutron detector 102B that includes a first ⁶Li active layer 112Adisposed at an intermediate point in the chamber 105 of the secondneutron detector 102B, in accordance with at least one embodiment of thepresent disclosure. As depicted in FIG. 5, only those tritons 168emitted from the lower surface of the first ⁶Li active layer 112A andthose tritons 168 emitted from the upper surface of the second ⁶Liactive layer 112B will enter the chamber 105A of the first neutrondetector 102A. In contrast, tritons 168 emitted from the upper surfaceand the lower surface of the ⁶Li active layer 112A will enter thechamber 105B of the second neutron detector 102B.

Advantageously, devices that read daughter particles such as tritons 168emitted by both sides of the ⁶Li active layer 112 may use less lithiumper measure of intrinsic efficiency as compared to detectors withsingle-sided particle detection, and thus may provide a beneficial costadvantage. Chart 510 depicts efficiency 512 for a single layer of activematerial 112 versus a double layer of active material 112 (along they-axis) against active layer thickness 514 (along the x-axis). Asdepicted in chart 510, the efficiency to detect thermal neutrons for adouble outward neutron detector 102A that uses two layers of activematerial 112A and 112B is about 20%, whereas a double inward neutrondetector 102B that uses only a single layer of active material 112A, andconsumes about 50% less 6Li than the double outward neutron detector102A, reaches about 25% efficiency. However, the increase in efficiencyof the double inward neutron detector 102B is offset by the addedconstruction complexity and cost of dual electrodes (one electrode oneach side of the layer of active material 112) as well as a loss inoperational ruggedness.

FIGS. 6A, 6B, 6C, and 6D depict an illustrative neutron detector 600suitable for use with the fissile neutron detection system 100 depictedin FIGS. 1A-1E, in accordance with one or more embodiments describedherein. The neutron detector 600 may include a top cover 104, a bottomcover 108, lithium-6 (⁶Li) foils 112A and 112B, a number of electrodes116 (collectively “electrodes 616”), a sidewall 120, a seal 124,isolators 128, front electronics board 632, and back electronics board636. The front electronics board 632 and back electronics board 636 mayinclude communicably coupled readout electronics or similar devices. Afirst ⁶Li foil 112A may be disposed proximate the top cover 104 and asecond ⁶Li foil 112B may be disposed proximate the bottom cover 108. Thetop cover 104 and the bottom cover 108 may be attached or otherwisedisposed proximate to the sidewalls 120 as shown in FIG. 6A to form achamber 105 that may, in operation, contain a readout gas. The seal 124provides a seal between the top cover 104, sidewalls 120, and bottomcover 108 that may greatly reduce or even prevent the readout gas fromescaping from the chamber 105. The seal 124 may also greatly reduce orprevent the entry of gases or fluids external to the chamber 105 fromentering the chamber 105. The electrodes 116 are fed through isolators128 that may be located on a front and back side of the sidewalls 120.The electrodes 116 may be electrically conductively coupled to the frontelectronics board 632 and may be conductively coupled to the backelectronics board 636. The readout electronics may provide a voltagebias between the electrodes 116 and the ⁶Li foils 112A and 112B. In someembodiments, each of the ⁶Li foils 112A and 112B may have a thickness ofabout 20 micrometers (μm) or less; about 50 μm or less; about 75 μm orless; about 100 μm or less; about 125 μm or less; or about 150 μm orless. Additionally, the readout electronics may decouple the signalsreceived from the electrodes 116, may amplify the signals received fromthe electrodes 116, and host post-digitization and further computer andwireless interfacing to share information relating to the collectedsignals with one or more user applications.

Upon exposure to fissile material such as plutonium and highly enricheduranium (HEU), neutrons and gamma rays may impinge upon the fissileneutron detector system 100. Neutrons impinging the fissile neutrondetector system 100 may pass through one or more external moderators 202and/or one or more neutron moderators 150 prior to impinging on the topplate 104 or the bottom plate 108 of the neutron detector 600. Theexternal moderator 202 and/or the neutron moderators 150 may reduce theenergy level of the incident fissile neutron 160 (e.g., 100 keV to 10MeV) to the energy level of a thermal neutron 162 (e.g., 0.025 eV). Thethermal neutron 162 may be captured by one of the ⁶Li atoms in the ⁶Lifoils 112A and 112B. The capture of the thermal neutron by the ⁶Li atomresults in a lithium 7 atom that decays into two daughter particles, analpha particle 166 and a triton 168. The triton 168 and alpha particle166 travel in opposite directions, and dissipate energy as they travelthrough the ⁶Li foil 112A and 112B.

Upon exiting the ⁶Li foil, the triton 168 and/or the alpha particle 166may have sufficient kinetic energy to ionize atoms present in thereadout gas within the chamber 105. Electrons 172 liberated during theionization of readout gas may drift in the direction of the electrodes116 and the ions produced during the ionization of readout gas atoms maydrift in the direction of the ⁶Li foils 112A and 112B. Electrons 172that drift within a predetermined distance of roughly 5 times the radiusof the electrode 116 (i.e., the Townsend avalanche region) may encounteran electric field that accelerates the electrons 172 at a ratesufficient to cause further ionization of the readout gas. The furtherionization of the readout gas liberates additional electrons 172, whichmay drift toward the electrodes 116 and cause even further ionization ofthe readout gas. This process that occurs within the Townsend avalancheregion 176 is called gas multiplication. Ionized readout gas atomswithin the Townsend avalanche region that move towards the ⁶Li foils112A and 112B cause a movement of charge along the electrode 116.

The charge moving along the electrode 116 is collected by the readoutelectronics 140 and amplified with a pulse-mode, charge-sensitivepreamplifier to produce a voltage output signal 192. Pulse heightdiscrimination circuitry included with in the readout electronics 140then compares the voltage output signal to a first predeterminedthreshold and determines if a fissile neutron event has been detected(e.g., for a gas multiplication of roughly 100, and an amplificationcircuitry gain of 1 fC/mV, pulse heights greater than 250 keV mayindicate the occurrence of a fissile neutron event). In someembodiments, the false positive detection rate of fissile neutrons 160based on the first predetermined threshold may be less than 1×10⁻⁵ for agamma ray exposure rate of 100 mR/hr. A second predetermined threshold,lower than the first predetermined threshold, may also be established.Voltage output signals below the second predetermined threshold may bedeemed attributable to very low ionizing gamma ray events or movementsof charge in the neutron detector 600 induced by one or more othersources (e.g., thermal heat, radio frequency electromagnetic radiation,and changes in the relative position of the electrodes 116 and the ⁶Lifoils 112A and 112B. Voltage output signals below the firstpredetermined threshold and above the second predetermined threshold areindicative of gamma ray events. The detected rate of fissile neutrons160 and gamma rays impinging upon the detector can be used in radiationdetection methodologies (e.g., to detect the presence of plutonium orhighly enriched uranium).

The composition of the readout gas may advantageously remain relativelyconstant over time to avoid deterioration of the gamma ray and neutrondetection process. Changes greater than 1% in the composition of thereadout gas can affect the Townsend avalanche process. For example,nitrogen, oxygen, or water molecules that leak into the chamber do notionize as well as the argon gas in the amplification region near theelectrodes 116, and therefore may reduce the Townsend avalanche processnear the electrodes 116 when introduced into the readout gas. Thisreduces the ability of the readout electronics to distinguish betweennoise, gamma ray, and fissile neutron events, thereby decreasing theefficiency of the neutron detector 600.

In some embodiments, the seal 124 can be formed from one or moreelastomeric materials, such as polyisobutylene, to maintain the readoutgas composition within the chamber 105 over extended periods of time(e.g., 30 years). The seal 124 can conform to the region between the topcover 104 or bottom cover 108 and the sidewalls 120, filling any gapsdue to imperfections in the surface quality of the top cover 104, thebottom cover 108, and the sidewalls 120. In some embodiments, thesurface quality of the top cover 104, bottom cover 108, and sidewalls120 may be selected to generate uniform electric fields near theelectrodes 116 of the neutron detector 600, with no regard for sealingof the top cover 104, bottom cover 108, and sidewalls 120, since theelastomeric nature of the seal 124 can accommodate such fluctuations.

FIGS. 7A and 7B depict another illustrative neutron detector 700suitable for use with the fissile neutron detection system 100 depictedin FIGS. 1A-1E, in accordance with one or more embodiments describedherein. The neutron detector 700 includes similar elements as theneutron detector 600 depicted in FIGS. 6A-6D. The neutron detector 700may include an array of elongated structural members 750. The elongatedstructural members 750 may include a top side 764, a bottom side 768,and a web 772. In embodiments, the elongated structural members 750 mayextend between the top cover 104 and the bottom cover 108. In someembodiments, the elongated structural members 750 may provide structuralsupport that may reduce undesirable mechanical vibrations within theneutron detector 700. For example, a tungsten wire electrode 116 havinga length of about 100 centimeters (cm) and a diameter of about 30micrometer (μm), placed under approximately 250 g of tension has a firstvibrational frequency of about 200 Hz, which corresponds to significantvibrations typically generated by motor vehicles. Placing a singlestructural support 750 near the middle of the tungsten wire electrode116, increases the first vibrational frequency to about 420 Hz, therebyreducing vibrations induced by vehicular movement by a factor of 100 ormore. Reducing vibrations in radiation detectors with surface areasgreater than 1000 square centimeters (cm²) is advantageous because asthe surface area of the neutron detector 700 and the length of theelectrodes 116 is increased, the increased dimensions can lead tovibrations that may cause changes in the relative position of theelectrodes 116 and/or the active materials 112 disposed in the chamber105. Relative changes in position between the electrodes 116 and theactive materials 112 can cause movement of charge within the fissileneutron detection system 100. Such charge displacement within thefissile neutron detection system 100 may generate voltage output signals192 that are indistinguishable from gamma ray or neutron signals.

The elongated structural members 750 can reduce mechanical vibrations ofthe top cover 104 and the bottom cover 108 by providing a mechanicalconnection therebetween. For example, adding an elongated structuralsupport 750 at the center of a 1 square meter (m²) neutron detector 750may increases the resonance frequency of the electrodes 116 in theneutron detector 700 by a factor of two or greater and may reduce theamplitude of the vibrations by a factor of two compared to when the topcover 104 and/or bottom cover 108 are supported only at the edges by thesidewall 120. The shape of the elongated structural members 750 may beselected to minimize vibrations between the top cover 104 and the bottomcover 108 (e.g., the cross section of the elongate structural members250 can be a “T”, an “I”, an “L”, an “X”, or a “C”). In someembodiments, the cross section of the elongated structural members 750may be rectangular.

In embodiments, each of the electrodes 116 may pass through a slot orsimilar aperture that penetrates at least a portion of the elongatedstructural member 750 to reduce the vibration of the electrodes 116. Theslots or apertures can provide mechanical support for the electrodes116. In some embodiments, the slot or aperture may be located near aside or edge of the elongated structural member 750. An electrode 116traversing the chamber 105 can pass through multiple slots or apertures.The elongated structural members 750 may be positioned within thechamber 105 such that the slots or apertures alternate sides of theelongated structural members 750 as the electrode 116 traverses thechamber 105. For example, the electrode 116 may pass through a firstslot located on the right side of a first elongated structural member750A, a second slot located on the left side of a second elongatedstructural member 750B, and a third slot located on the left side of athird elongated structural member 750C. In some embodiments, some or allof the elongated structural members 250 may be fabricated using anelectrically non-conductive material. In some embodiments, the slot oraperture may be positioned to provide an upward or downward lateralforce on the electrode 116. In some embodiments, the electrodes 116 maybe supported by a structural member that attaches to the top cover 104or the bottom cover 108, but not both. In some embodiments, theelongated structural members 750 may contact the top cover 104 and thebottom cover 108 but do not include a slot or aperture and are displacedfrom the electrodes 116 so as to not cause a mechanical interference.

FIG. 8 is a high-level flow diagram of a method 800 for detectingfissile neutrons 160 using a fissile neutron detector 100 that includesat least one neutron detector 102, and at least one neutron moderator150 disposed proximate the at least one neutron detector 102, inaccordance with at least one embodiment of the present disclosure.High-energy fissile neutrons such as those emitted by plutonium andhighly enriched uranium (HEU) provide a tell-tale indicator of thepresence of such materials. Fissile neutrons can have energy levels thatexceed 100 keV. At such energy levels, a large percentage of fissileneutrons may pass undetected through the active material 112 typicallyfound in neutron detectors 102, 600, and 700. The presence of a neutronmoderator, such as the neutron moderator 150, can beneficially reducethe energy level of fissile neutrons to the energy level of thermalneutrons, about 0.025 eV. Such a reduction in energy level may be atleast partially attributable to collisions between the fissile neutrons160 and hydrogen nuclei in the moderator 150. Consequently, moderatorshaving a high percentage of hydrogen by weight may be preferable.Thermal neutrons 162 may impact the active material 112, causing thespontaneous formation of charge-carrying daughter particles, such asalpha-particles 166 and tritons 168. The triton 168 can ionize a gaswithin the chamber 105 of the neutron detector 102, 600, 700. Thepresence of the ionized gas and dissociated electrons 172 within thechamber 105 can induce a current flow on an electrode in the chamberthat is maintained at a potential. The current flow may be proportionalto the number or rate at which fissile neutrons 160 are impacting theactive material 112 within the chamber 105. The method 800 commences at802.

At 804, the energy level of at least a portion of the fissile neutrons160 incident upon the fissile neutron detection system 100 is reduced tothe energy level of a thermal neutron 162. In some implementations, thisreduction in energy level is accomplished using the at least one neutronmoderator 150. Such neutron moderators 150 may include a number ofinterstitial neutron moderators 150 that are positioned proximate afirst neutron detector 102A and second neutron detector 102B and withina space formed between the between the first neutron detector 102A andsecond neutron detector 102B. Such neutron moderators 150 may includeone or more neutron moderators 150 having an exterior surface and inwhich at least a portion of the exterior surface is disposed proximatethe at least one neutron detector 102. Such neutron moderators mayadditionally or alternatively include one or more external neutronmoderators 202 positioned proximate an exterior surface of the firstneutron detector 102A, the second neutron detector 102B, and/or theintermediate neutron detectors 150A-150 n.

The one or more neutron moderators 150 reduce the energy level of theincident fissile neutrons 160 may include one or more solids, liquids,and/or compressed gases capable of reducing the energy level of at leastsome of the incident thermal neutrons 160. In some implementations, theone or more neutron moderators 150 may include materials, compounds, orsubstances having a significant hydrogen concentration—greater thanabout 30 weight percent hydrogen; greater than about 40 weight percenthydrogen; greater than about 50 weight percent hydrogen; or greater thanabout 60 weight percent hydrogen. It is believed the impact between theincident fissile neutrons 160 and the hydrogen nuclei within the one ormore neutron moderators 150 may reduce the energy level of the incidentfissile neutron 160 to that of a thermal neutron 162 which then exitsthe one or more neutron moderators 150.

Due to the random nature of the collisions within the one or moreneutron moderators, a portion of the incident fissile neutrons 160 mayflow as neutrons having an energy level at or above that of a thermalneutron 162 from the one or more neutron moderators 150 in a directionthat precludes impingement on a neutron detector 102, 600, 700 disposedproximate at least a portion of the exterior surface of the neutronmoderator 150. For example, an incident fissile neutron 160 may flowfrom the “side” or “edge” of the one or more neutron moderators 150 in adirection along a vector pointing away from a neutron detector 102, 600,700 that is proximate at least a portion of the surface of the one ormore neutron moderators 150. The geometry of the fissile neutrondetection system 100, the geometry and composition of the neutronmoderator 150, the geometry and composition of the external neutronmoderator 202, and the construction and geometry of the neutron detector102, 600, 700 all play a role in determining the capture rate ofincident fissile neutrons 160. For example, planar neutron detectorssuch as the neutron detector 102 depicted in FIGS. 1A-1E, the neutrondetector 600 depicted in FIGS. 6A-6D, and the neutron detector 700depicted in FIGS. 7A-7B all present a significantly increasedcross-sectional (or fissile neutron capture) area which provides amarked advantage and improvement in fissile neutron detectionperformance and accuracy over straw-type neutron detectors in whichdetector “straws” may be positioned within a block of neutron moderator.

In some implementations, the physical configuration of the one or moreneutron moderators 150 and the one or more neutron detectors 102, 600,700 may be such at a minimum of about 50% or more; about 55% or more;about 60% or more; about 65% or more; about 70% or more; about 75% ormore; about 80% or more; about 85% or more; or about 90% or more of thethermal neutrons 162 exiting the one or more neutron moderators 150impinge upon, strike, or otherwise enter the one or more neutrondetectors 102, 600, 700.

At 806, at least some of the thermal neutrons 162 exiting the one ormore neutron moderators 150 may pass through the top cover 104 or bottomcover 108 of the first neutron detector 102A and enter the chamber 105Aor pass through the top cover 104 or bottom cover 108 of the secondneutron detector 102B and enter the chamber 105B. Once inside of thechamber 105, the thermal neutron 162 may impinge on one or more activematerials 112 disposed therein. An active material 112 may include anysubstance, isotope, element, compound, or mixture capable of generatingcharge-carrying particles upon exposure to thermal neutrons 162.Non-limiting examples of such active materials include, but are notlimited to, lithium-6 (⁶Li); boron-10 (¹⁰B); and helium-3 (³He). Suchactive materials 112 may be present in the chamber 105 in one or moreforms. For example, in some implementations ⁶Li in the form of thin (50μm to 150 μm) sheets may provide all or a portion of the active material112 that are disposed either at one or more intermediate points (e.g.,neutron detector 102) or proximate one or more interior surfaces (e.g.,neutron detector 600) of the chamber 105. In some implementations, ¹⁰Bin the form of a thin layer disposed on at least a portion of theinterior surface of the chamber 105 may provide all or a portion of theactive material 112. In some implementations, ³He in the form of a gasdisposed in the chamber 105 may provide all or a portion of the activematerial 112.

The charge-carrying particle(s) emitted by the active material 112 inresponse to the impact of the thermal neutron 162 may travel into thereadout gas 170 disposed within the chamber 105. The charge-carryingparticles, such as a triton 168, may ionize a portion of the readout gas170, creating a positively charged readout gas ion and an electron 172.

At 808, the neutron detector 102, 600, 700, in response to the chargedparticles generated by the impact of the thermal neutron 162 on theactive material 112, generates a current indicative of a number ofthermal neutrons 162 that impact the active material 112 or a rate atwhich thermal neutrons 162 impact the active material 112 in therespective neutron detector 102, 600, 700. The method 800 concludes at810.

FIG. 9 is a high-level flow diagram of a method 900 for generating acurrent in a neutron detector 102, 600, 700 in response to the impact ofthe thermal neutron 162 on an active material such as ⁶Li or ¹⁰Bdisposed in the chamber 105 of the neutron detector 102, 600, 700, inaccordance with at least one embodiment of the present disclosure. Theinteraction of the charged particles, such as the triton 168, generatedby the impact of the thermal neutron 162 on the active material 112,with a readout gas 170 disposed in the chamber 105 can cause a currentto flow on an electrode 116 placed in the chamber 105. The electrode 116may be maintained at a potential that differs from the potential of theactive material 112. The method 900 commences at 902.

At 904, one or more charged particles may be generated by the capture ofthe thermal neutron 162 by the active material 112. In implementationsusing ⁶Li, these charged particles may include an alpha-particle 166(two protons and two neutrons) and a triton 168 (one proton and twoneutrons). In implementations, the triton 168 may travel a distance ofup to 135 μm through a ⁶Li sheet of active material 112. Thus, withinneutron detectors 102, 600, 700 using a ⁶Li active materials, thethickness of a ⁶Li sheet of active material 112 may be maintained atless than 135 μm to increase the probability that the triton 168 willescape the active sheet 112.

In implementations using ¹⁰B, these charged particles may include analpha particle and a ⁷Li ion. About 78% of the time either of the alphaparticle or the ⁷Li ion may escape a 1 μm thick layer of ¹⁰B. Thus,within detectors 102, 600, 700 using ¹⁰B active materials, the ¹⁰B istypically applied as a coating or layer to all or a portion of theinterior surfaces of the chamber 105.

At 906, the charged particles escaping the active material 112 ionize atleast a portion of a readout gas 170 disposed within the chamber 105. Insome implementations, the readout gas 170 may include an elemental gas,a gas mixture, a gas combination, a gas compound, or any othercombination of gases. In some implementations, the readout gas 170 mayinclude one or more noble gases, such as argon (Ar). In ⁶Liimplementations, at least a portion of the alpha particles 166 and/or atleast a portion of the tritons 168 may ionize a portion of the readoutgas 170, generating drift electrons 172 and a positively charged readoutgas ion. In ¹⁰B implementations, at least a portion of the alphaparticles 166 and/or at least a portion of the ⁷Li particles may ionizea portion of the readout gas 170, generating drift electrons 172 and apositively charged readout gas ion.

At 908, an electrode 116 placed in the chamber 105 may be maintained ata potential that differs from the potential of the active material 112.In some instances, the electrode 116 may be maintained at a potentialthat is positive (e.g., +100 V) measured with respect to the potentialof the active material 112 (e.g., grounded or 0 V). The electric fieldcreated within the chamber 105 may cause drift electrons 172 to drift ortravel towards the electrode 116. The electric field created within thechamber 105 may also cause the positively charged readout gas ions todrift or travel towards the active material 112. As the drift electrons172 travel and/or accelerate toward the electrode 116, additionalionization of the readout gas 170 may occur. This “chain reaction” ofionization of the readout gas 170 may, in turn, cause an avalanche ofelectrons 174 within an amplification region 176 about the electrode116.

At 910, the combined flow of positively charged readout gas ions towardthe active material 112 and the flow of drift electrons 172 toward theelectrode 116 causes an overall charge flow within the chamber 105. Thisflow of charges within the chamber 105 may induce a current in theelectrode 116. In some instances, the magnitude of the current in theelectrode may be indicative of the number of thermal neutrons 162 thatimpact the active material 112 and/or the rate at which thermal neutrons162 impact the active material 112. The method 900 concludes at 910.

FIG. 10 is a high-level flow diagram of a method 1000 for generating acurrent in a neutron detector 102, 600, 700 in response to the impact ofthe thermal neutron 162 on an active material such as ³He or borontrifluoride (BF₃) disposed in the chamber 105 of the neutron detector102, 600, 700, in accordance with at least one embodiment of the presentdisclosure. The interaction of the charged particles generated by theinteraction between a thermal neutron 162 and a gaseous active material112 such as ³He disposed in the chamber 105 may cause a current to flowon an electrode 116 disposed within the chamber 105. In some instances,the electrode 116 may be maintained at a potential that differs from thepotential elsewhere in the chamber 105 and different from the potentialof the gaseous active material 112. The method 1000 commences at 1002.

At 1004, one or more charged particles may be generated by the captureof the thermal neutron 162 by the active material 112. Inimplementations using ³He, these charged particles may include a protium(a hydrogen isotope containing a single proton) and tritium (a hydrogenisotope containing a single proton and two neutrons).

At 1006, an electrode 116 placed in the chamber 105 may be maintained ata potential that differs from the potential of the active material 112.In some instances, the electrode 116 may be maintained at a potentialthat is positive (e.g., +100 V) measured with respect to the potentialof the active material 112 (e.g., grounded or 0 V). The electric fieldcreated within the chamber 105 may cause the charged particles to driftor travel towards the electrode 116.

At 1008, the flow of charged particles toward the electrode 116 causesan overall charge flow within the chamber 105. This flow of chargeswithin the chamber 105 may induce a current in the electrode 116. Insome instances, the magnitude of the current in the electrode may beindicative of the number of thermal neutrons 162 that impact the activematerial 112 and/or the rate at which thermal neutrons 162 impact theactive material 112. The method 1000 concludes at 1010.

FIG. 11A depicts a cross-sectional elevation of an illustrative neutrondetector 102 and neutron moderator 150 arrangement 1100 that may be usedin one implementation of a fissile neutron detection system 100, inaccordance with at least one embodiment of the present disclosure. Inthe implementation depicted in FIG. 11A, the neutron detector 102 isformed into a ring-like structure that at least partially surrounds theneutron moderator 150. In such an arrangement, the incident high-energyfissile neutrons 160 may pass through the neutron detector 102 andimpinge upon the neutron moderator 150. Within the neutron moderator150, the energy level of at least some of the fissile neutrons 160 maybe reduced to an energy level of a thermal neutron 162. At least aportion of the thermal neutrons 162 may exit the neutron moderator 150and enter the neutron detector 102.

FIG. 11B depicts a cross-sectional elevation of an illustrative neutrondetector 102 and neutron moderator 150 arrangement 1100 that may be usedin one implementation of a fissile neutron detection system 100, inaccordance with at least one embodiment of the present disclosure. Inthe implementation depicted in FIG. 11B, a number of neutron detectors102A-102D at least partially surround an exterior surface of the neutronmoderator 150. In such an arrangement, the incident high-energy fissileneutrons 160 may pass through one of the number of neutron detectors102A-102D and impinge upon the neutron moderator 150. Within the neutronmoderator 150, the energy level of at least some of the fissile neutrons160 may be reduced to an energy level of a thermal neutron 162. At leasta portion of the thermal neutrons 162 may exit the neutron moderator 150and enter one of the number of neutron detectors 102A-102D.

FIG. 11C depicts a cross-sectional elevation of an illustrative neutrondetector 102 and neutron moderator 150 arrangement 1100 that may be usedin one implementation of a fissile neutron detection system 100, inaccordance with at least one embodiment of the present disclosure. Inthe implementation depicted in FIG. 11C, a number of neutron detectors102A-102B at least partially surround an exterior surface of the neutronmoderator 150. Although depicted as a planar body, in such anarrangement, each of the neutron detectors 102 may include a planarbody, a curved body, or an angular body. As depicted in FIG. 11C, afirst neutron detector 102A may be spaced a first distance 1102 from atleast a portion of an exterior surface of the neutron moderator 150 anda second neutron detector 102B may be spaced a second distance 1104 fromat least a portion of the exterior surface of the neutron moderator 150.

FIG. 11D depicts a cross-sectional elevation of an illustrative neutrondetector 102 and neutron moderator 150 arrangement 1100 that may be usedin one implementation of a fissile neutron detection system 100, inaccordance with at least one embodiment of the present disclosure. Inthe implementation depicted in FIG. 11D, a neutron moderator 150 atleast partially surrounds at least a portion of an exterior surface of afirst neutron detector 102A and an exterior surface of a second neutrondetector 102B.

FIG. 11E depicts a cross-sectional elevation of an illustrative neutrondetector 102 and neutron moderator 150 arrangement 1100 that may be usedin one implementation of a fissile neutron detection system 100, inaccordance with at least one embodiment of the present disclosure. Inthe implementation depicted in FIG. 11E, a first neutron detector 102Aand a second neutron detector 102B are disposed in an alternating or“sandwich” arrangement with a first neutron moderator 150A and a secondneutron moderator 150B. As depicted in FIG. 11E, at least a portion ofthe surface of the first neutron detector 102A may be exposed, while asimilar portion of the surface of the second neutron detector 102B isdisposed proximate the second neutron moderator 150B.

FIG. 11F depicts a cross-sectional elevation of an illustrative neutrondetector 102 and neutron moderator 150 arrangement 1100 that may be usedin one implementation of a fissile neutron detection system 100, inaccordance with at least one embodiment of the present disclosure. Inthe implementation depicted in FIG. 11F, a first neutron detector 102Aand a second neutron detector 102B are disposed in an alternating or“sandwich” arrangement with a first neutron moderator 150A, a secondneutron moderator 150B, and a third neutron moderator 150C.

FIG. 11G depicts a cross-sectional elevation of an illustrative neutrondetector 102 and neutron moderator 150 arrangement 1100 that may be usedin one implementation of a fissile neutron detection system 100, inaccordance with at least one embodiment of the present disclosure. Inthe implementation depicted in FIG. 11G, a first neutron detector 102Aand a second neutron detector 102B are disposed in an alternating or“sandwich” arrangement with a non-planar neutron moderator 150. In suchan embodiment, the neutron moderator 150 may have any shape, geometry,or physical form.

FIG. 11H depicts a cross-sectional elevation of an illustrative neutrondetector 102 and neutron moderator 150 arrangement 1100 that may be usedin one implementation of a fissile neutron detection system 100, inaccordance with at least one embodiment of the present disclosure. Inthe implementation depicted in FIG. 11H, six neutron detectors 102A-aredisposed about at least a portion of an exterior surface of a non-planarneutron moderator 150. In such an arrangement, some or all of theneutron detectors 102 may have a contoured body configured to closelyapproximate the surface contour of at least a portion of the exteriorsurface of the non-planar neutron moderator 150.

FIG. 11I depicts a cross-sectional elevation of an illustrative neutrondetector 102 and neutron moderator 150 arrangement 1100 that may be usedin one implementation of a fissile neutron detection system 100, inaccordance with at least one embodiment of the present disclosure. Inthe implementation depicted in FIG. 11I, six neutron detectors 102A—aredisposed about at least a portion of an exterior surface of a non-planarneutron moderator 150. In such an arrangement, some or all of theneutron detectors 102 may have a planar body disposed about at least aportion of the exterior surface of the non-planar neutron moderator 150.

FIG. 11J depicts a cross-sectional elevation of an illustrative neutrondetector 102 and neutron moderator 150 arrangement 1100 that may be usedin one implementation of a fissile neutron detection system 100, inaccordance with at least one embodiment of the present disclosure. Inthe implementation depicted in FIG. 11J, a number of neutron detectors102A-102B at least partially surround an exterior surface of a pluralityof neutron moderators 150A-150D. Although depicted as a planar body, insuch an arrangement, each of the neutron detectors 102 may include aplanar body, a curved body, or an angular body.

FIG. 11K depicts a cross-sectional elevation of an illustrative neutrondetector 102 and neutron moderator 150 arrangement 1100 that may be usedin one implementation of a fissile neutron detection system 100, inaccordance with at least one embodiment of the present disclosure. Inthe implementation depicted in FIG. 11K, a number of neutron detectors102A-102H at least partially surround an exterior surface of a neutronmoderator 150. Although depicted as a planar body, in such anarrangement, each of the neutron detectors 102A-102H may include aplanar body, a curved body, or an angular body.

It is noted that the configurations depicted in FIGS. 11A-11K employinga neutron detector arrangement in which the neutron detector 102, 600,700 generally surrounds the neutron moderator 150 may providesignificant advantages over prior neutron detector designs. Such priorneutron detector designs may be classified as either TYPE I arrangementsin which the moderator surrounds the detector and TYPE II arrangementsin which the moderator is interspersed among an array of detectors thatare separated from each other by substantial amounts along variousdirections.

With regard to TYPE I arrangements, back scattered neutrons thatinitially strike the moderator may be directed away from the innerdetector such that the backscattered neutrons are lost to detection.With respect to the configurations disclosed herein, it is noted thatthe great majority of such backscattered neutrons may be collected bythe neutron detectors 102, 600, 700 as a consequence of the manner inwhich the detector arrangement may directly surround the moderator inalmost all directions.

With regard to TYPE II arrangements, it is noted that the disclosedembodiments advantageously reduce the distance that backscatteredneutrons travel prior to impacting a neutron detector. In contrast, evenoptimized TYPE II arrangements may be handicapped by the distancesbackscattered neutrons must travel prior to detection. While TYPE IIsystems may allow for an increasing number of neutron detectors as a wayof increasing overall system efficiency, such additional detectorsgenerally push the system as a whole toward a heavier, bulkier, and moreexpensive solution than the embodiments described herein. The systemgeometries disclosed herein offer significant improvements in space,weight and cost when compared to traditional TYPE II systems.

In reference to FIGS. 11A-11K, for purposes of descriptive clarity,attention is directed to several aspects of performance as delineatedfrom prior technical discussions. The detector systems of the presentdisclosure are arranged such that a neutron moderator 150 is surroundedby a neutron detector 102, 600, 700 arrangement such that incomingfissile neutrons 160 generally pass through the neutron detector 102,600, 700 before striking the neutron moderator 150, and the vastmajority (>60%) of all thermal neutrons 162 scattered by the neutronmoderator 150 (including back scattered as well as forward scatteredthermal neutrons) will be collected by the at least one neutron detector102, 600, 700. Furthermore, in any of the described embodiments, themoderator may, in some implementations, define a generally planargeometry (not necessarily a flat plane) having a large area (anywherefrom 0.5 m² to 100 m²) and a thickness that is small compared with anygiven lateral extent thereof, and the at least one neutron detector 102,600, 700 surrounds the neutron moderator 150 in close proximityespecially over the major planar surfaces. Thus the fissile neutrondetection system 100 may be considered as a layered arrangement thatprovides for advantages over prior neutron detection systems.

Both forward and backward scattered neutrons both travel only shortdistances before impinging on the neutron detectors 102, 600, 700. Thisdetector characteristic—that of short scattering to detectionpaths—helps insure that forward and backwards scattered thermal neutrons162 tend not to be absorbed by intervening materials and thus can beentirely lost to detection. Furthermore, at least one further benefit ofthe short path is that it takes up less linear space that a long pathwould require. Applicants appreciate that at least in the cases offorward and backward scattering, the short path between scattering anddetection provides for fissile neutron detector systems that have lowerextent (at least in the direction of initial neutron trajectory) ascompared to conventional detectors. It is to be noted that the termsforward and backward scattering as employed herein can be considered asany scattering event where the scattered neutron deviates from itsinitial incoming trajectory by more than approximately 60 degrees. Whilethe benefits of the described approaches are clearly not limited toplanar embodiment, applicants are unaware of any conventional systemthat uses solid neutron conversion materials that achieves as comparablyhigh efficiency (50% detection of fissile material) within thicknessranges that are so short as compared to lateral extent, withoutincreasing by 25% or more the amount of lithium or boron that is used inthe system. In other words, conventional approaches require detectorsystems of significantly greater thickness or neutron conversionmaterial (such as lithium or boron) as compared to those disclosedherein. In the context of commercial applications wherein size, weightand cost are paramount, this advantage represents a significantimprovement.

Summarizing with respect to overall operation of various embodimentdescribed above, a moderator arrangement, composed of a moderatormaterial, can be surrounded by a detector arrangement such that at least60% scattered neutrons that exit the moderator travel only a shortdistance before they strike an active area (for example lithium foil) ofthe detector arrangement.

In some embodiments the moderator defines a generally planer shapehaving a thickness that is short compared with any lateral extentthereof. It is noted that in the context of this disclosure there is norequirement that these planar geometries be flat, and it should beappreciated that the planar geometries described herein could be curvedin a variety of ways just as any piece of sheet metal or paper can becurved and bent in a variety of ways and yet still regarded as beinggenerally planar. At least in the case of generally planar moderator anddetector geometries, the overall collection efficiency tends to exceedthat which can be obtained in conventional systems such as the TYPE Iand TYPE II systems for the same amount of conversion material used inthe detector. In additional to higher absolute efficiencies, therelative efficiency, and reduction of weight, cost, and/or thicknesstends to exceed that of conventional detector systems such as the TYPE Iand TYPE II conventional systems described above. This aspect can bereadily appreciated by comparing the described planar embodiment withconventional detectors constrained to occupy and be contained within thesame or similar spatial envelope as Applicant's systems. For example adetector system such as that of FIG. 1 occupies a spatial area of 1 m²and a thickness of 0.15 meters. A conventional TYPE I detector ofsimilar shape could readily lose between 25% and 40% of efficiency ascompared to a similar sized unit that is constructed based on thisdisclosure. Similarly, a TYPE II system of similar area may require athickness of 0.2 meters or more and need 200% or more lithium or boronmaterial and would thus be at least 25% times heavier and far moreexpensive.

With continued focus on generally planar embodiments, Applicants notethat certain ones of the above configurations can include an outermoderator (proximate to and not surrounded by the detector) and an innermoderator that is almost entirely surrounded by the detectorarrangement. In this context, Applicants consider a distinguishingfeature of some of the embodiments disclosed herein that only incomingneutrons entering from extremely shallow sideways angles (for example insome embodiments only neutrons entering sideways with less than 20degrees from plane defined by the planar modulator) can strike the innermoderator without first passing through a detector. (Since the detectorsare generally not intended to have high efficiency for sideways incidentneutrons these neutrons may be of little consequence at least in manyintended applications.) While this feature by itself does not directlyresult in the dislodged efficiency improvements, it is to be noted thatinsofar as all or most impinging high energy neutrons cannot enter theinner moderator without passing through the detector, it is converselythe case that all or most scattered low energy neutrons cannot exit andpass away from the moderator without passing through the detectors. Thislatter consideration clearly results sweeping advantages compared to theconventional systems, including but not limited to Type 1 and Type twosystems, and to whatever extent the former consideration results inand/or is related to these benefits, it is considered by Applicants atthe very least to be of general interest.

Of the many benefits of the disclosed systems, it is again of particularinterest that the scattered neutrons can be collected with a relativelysmall amount of intervening structure at least as compared withconventional arrangements in which large distances and substantialamounts of material may be present in the intervening spatial extentlying between a given moderator and an associated detector. Oneimplication of this unusual feature is that in many embodiments, thedesigner is free to surround the moderator in very close proximity. Forexample, for a moderator of a given thickness, in many cases thedisclosed embodiments allow for the detector to surround the moderatorwith gap spacing therebetween that are much smaller than the moderatorthickness. This is of benefit at least for the reason that such closemoderator detector spacing, over the great majority of the moderatorsurface, affords very little opportunity of escape for scatteredneutrons. In other words the lose moderator detector spacing, overalmost the entirety of the moderator, prevents most scattered neutronsfrom escaping the detector system without impinging on some part of thedetector. Applicants are simply unaware of any conventional systems thatcan reasonably be regarded as sharing this important feature.Summarizing with respect to the foregoing paragraph, many of thedescribed detector arrangements surround their associated innermoderator with moderator-detector spacing that are small as comparedwith moderator thickness.

FIG. 12A depicts an exploded view of another illustrative neutrondetector 1200 that may be used alone or in combination in the fissileneutron detection systems 100, 600, 700, in accordance with at least oneembodiment described herein. The neutron detector uses a modularassembly 1201 in which components such as a first ground plate 1202A, afirst set of electrodes 116A, the active material 112 (and any support106—not shown) a second set of electrodes 116B, and a second groundplate 1202B may be preassembled prior to disposal in a housing 1208 thatincludes the top cover 104, the bottom cover 108 and at least a portionof the sidewalls 120.

In some implementations, the first ground plate 1202A, a first set ofelectrodes 116A, the active material 112 a second set of electrodes116B, and a second ground plate 1202B may be preassembled using a numberof internal spacers to provide clearance between the electrodes 116, theactive material 112, and the ground plates 1202. The internal spacersmay include a number of side spacers 1204A-1204D (collectively, “sidespacers 1204”) and a number of end spacers 1206A-1206D (collectively,“end spacers 1206”) that, when assembled provide sufficient clearanceand electrical isolation of the various components within the within themodular assembly.

As used herein terms such as “side” and “end” denote locations relativeto each other and do not represent absolute references. Thus, an “endobject” may function as a “side object” when the object is rotatedthrough an angle such as 90 degrees. Similarly, a “side object” mayfunction as an “end object” when the object is rotated through an anglesuch as 90 degrees.

The ground plates 1202 can include one or more electrically conductivematerials. Such materials may include one or more metals such asaluminum, copper, or alloys containing various quantities of aluminum orcopper. In some implementations, the ground plates 1202 may include aconductive mesh material to permit the passage of the readout gas 170through all or a portion of the ground plates 1202. In someimplementations, the ground plates 1202 may include one or moreelectrically insulative materials disposed on all or a portion of theexterior surface of the ground plate 1202 proximate the housing 1208.

The side spacers 1204 may include any number or combination of devicesor components capable of maintaining a desired separation between theactive material 112 and a ground plate 1204. The side spacers 1204 mayhave any shape, and thus although shown as straight members in FIG. 12A,the side spacers 1204 may be curved, arced, angular or any other shapeneeded to maintain the desired separation or distance between the activematerial 112 and the ground plate 1202.

The electrodes 116 are terminated on a number of ganging boards or buses1210A-1210D (collectively “buses 1210”). The buses 1210 advantageouslyprovide distribution of electric power and collection of current signalsvia a limited number of penetrations through the neutron detector 1200.For example, as depicted in FIG. 12A, the buses 1210 permit the gangingof electrode power and beneficially route all electrical connectionsthrough one or more couplers 1212. In some instances the one or morecouplers 1212 may include a modular plug or similar device thatsimplifies and speeds electrical connection of the respective fissileneutron detection system 1200. In embodiments, the one or more couplers1212 may include a number of conductors for powering the electrodes 116within the fissile neutron detection system 1200. In embodiments, theone or more couplers 1212 may include a number of signal conductors forcommunicating fissile neutron detection signals to one or more externaldevices, such as a count readout device and/or alarm device.

The use of one or more couplers 1212 may greatly reduce the number ofpenetrations through the housing 1208. Reducing the number ofpenetrations through the neutron detector housing reduces the likelihoodof egress of the readout gas 170 from the chamber 105 and also reducesthe likelihood of ingress of environmental contaminants into the chamber105.

The housing 1208 may include all or a portion of the top cover 104, thebottom cover 108, and at least a portion of one or more sidewalls 120A.Advantageously, the housing 1208 may be cast, extruded or similarlyformed using a single component, thereby limiting the number of jointsin the neutron detector 1200. Minimizing the number of joints within theneutron detector 1200 beneficially reduces the likelihood of egress ofthe readout gas 170 from the chamber 105 and also reduces the likelihoodof ingress of environmental contaminants into the chamber 105. In someimplementations, the end plates 120B and 120C may be attached to thehousing 1208 using one or more joints having a sealant 124 disposedtherein. In some implementations, the end plates 120B and 120C may beattached to the housing 1208 via welding or brazing. In otherembodiments, the end plates 120B and 120C may be attached to the housing1208 via one or more fasteners, such as one or more screws or similar.

FIG. 12C is a detail drawing depicting an electrode connection device1250 for use with the illustrative neutron detector 1200 depicted inFIGS. 12A and 12B, in accordance with at least one embodiment of thepresent disclosure. In at least some implementations the electrodes 116may electrically conductively couple to a bus ganging structure or bus1210. One or more conductors or pins 1256 may pass through a sealingplate 1258 that is affixed to the housing 1208, for example to the topcover 104 of the housing 1208. One or more seals 1260, for example oneor more polyisobutylene seals 1260 may be disposed between the sealingplate 1258 to provide a hermetically sealed chamber 105. In someinstances a member or standoff 1252 may separate and, in some instances,electrically isolate the bus 1210 from the sealing plate 1258. In theembodiment depicted in FIG. 12C, a conductive bolt 1254 penetrates thesealing plate 1258 and at least partially surrounds the pin 1256. Asecond polyisobutylene seal 1262 may be disposed between the conductivebolt 1254 and the sealing plate 1258.

FIG. 12D is a detail drawing depicting another electrode connectiondevice 1250 for use with the illustrative neutron detector 1200 depictedin FIGS. 12A and 12B, in accordance with at least one embodiment of thepresent disclosure. In at least some implementations, some or all of theelectrodes 116 may electrically conductively couple to a bus gangingstructure or bus 1210. One or more conductors or pins 1256 pass througha sealing plate 1258 that is affixed to the housing 1208, for example tothe top cover 104 of the housing 1208. One or more seals 1260, forexample one or more polyisobutylene seals 1260 may be disposed betweenthe sealing plate 1258 to provide a hermetically sealed chamber 105. Insome instances a member or standoff 1252 may separate and, in someinstances, electrically isolate the bus 1210 from the sealing plate1258. In the embodiment depicted in FIG. 12D, a sealing member 1270,such as an epoxy sealing member, penetrates the sealing plate 1258 andat least partially surrounds the pin 1256.

FIG. 12E is a detail drawing depicting another electrode connectiondevice 1250 for use with the illustrative neutron detector 1200 depictedin FIGS. 12A and 12B, in accordance with at least one embodiment of thepresent disclosure. In at least some implementations, some or all of theelectrodes 116 may electrically conductively couple to a bus gangingstructure or bus 1210. One or more conductors or pins 1256 pass throughan aperture in the housing 1208, for example to the top cover 104 of thehousing 1208. One or more a weld or O-ring seal may be disposed to sealat least a portion of the aperture in the housing 1208. In someinstances, a plug 1286 such as a metal or polymeric plug may be disposedproximate the aperture in the housing 1208. A glass or epoxy seal 1284may at least partially surround the pins 1256 extending from the bus1210. A member or standoff 1252 may separate and, in some instances,electrically isolate the bus 1210 from the sealing plate 1258. In theembodiment depicted in FIG. 12E, the combination of the weld or O-ringseal 1280, the metal or polymeric plug 1286, and the glass or epoxy seal1284 at least partially surrounds the pin 1256 and provides a hermeticseal for the chamber 105.

FIG. 12F is a close up perspective view of the electrode connectiondevice 1250 depicted in FIG. 12C, in accordance with at least oneembodiment of the present disclosure. FIG. 12G is a close up plan viewof the electrode connection device 1250 depicted in FIG. 12C, inaccordance with at least one embodiment of the present disclosure.

FIG. 13A is an exploded view of an illustrative fissile neutrondetection system 1300 that uses three neutron moderators 150A-150C andeight neutron detectors 1200 such as depicted in FIGS. 12A-12F, inaccordance with at least one embodiment of the present disclosure. FIG.13B is an assembled view of the illustrative fissile neutron detectionsystem 1300 depicted in FIG. 13A, in accordance with at least oneembodiment of the present disclosure. The modular construction of thefissile neutron detection system 1200 beneficially permits positioningof any number of systems 1200 in a variety of configurations. Such may,for example, facilitate the use of a single fissile neutron detectionsystem 1200 within a relatively compact, portable, handheld device andthe combination of a number of fissile neutron detection system 1200into a stationary roadside monitoring array. As depicted in FIGS. 13Aand 13B, a neutron moderator 150A is disposed between eight fissileneutron detection systems 1200A-1200H arranged in two rows of fourdetectors. The majority of the surface area of the neutron moderator150A is therefore disposed proximate one or more neutron detectors 1200.External neutron moderators 150B and 150C are disposed proximate thesurfaces of the fissile neutron detectors 1200A-1200H that are oppositeneutron detector 150A. As depicted in FIGS. 13A and 13B the fissileneutron detection systems 1200 may be arranged such that the one or morecouplers 1212A-1212H for each of the systems 1200A-1200H, respectively,exits the assembly from a single end. Such an arrangement may facilitatethe connection of each of the fissile neutron detection systems 1200 toa communications bus a power distribution bus, or any combinationthereof.

In embodiments, the chamber 105 of the neutron detector 1200 may have alength, measured along a first axis, of about 10 centimeters (cm) orgreater; about 20 cm or greater; about 30 cm or greater; about 50 cm orgreater; about 75 cm or greater; about 100 cm or greater; about 200 cmor greater; about 500 cm or greater; about 700 cm or greater; or about1000 cm or greater. In embodiments, the chamber 105 may have a height,measured along a second axis orthogonal to the first axis, of about 0.5centimeters (cm) or less; about 1 cm or less; about 1.5 cm or less;about 2 cm or less; about 2.5 cm or less; about 3 cm or less; about 3.5cm or less; about 4 cm or less; about 4.5 cm or less; or about 5 cm orless. In embodiments, the chamber 105 may have a width, measured along athird axis orthogonal to the first axis and the second axis, of about 10centimeters (cm) or less; about 15 cm or less; about 20 cm or less;about 25 cm or less; about 30 cm or less; about 35 cm or less; about 40cm or less; about 45 cm or less; about 50 cm or less; or about 100 cm orless.

In some implementations, the physical configuration of the one or moreneutron moderators 150 and the one or more neutron detectors 1200 may besuch at a minimum of about 50% or more; about 55% or more; about 60% ormore; about 65% or more; about 70% or more; about 75% or more; about 80%or more; about 85% or more; or about 90% or more of the thermal neutrons162 exiting the one or more neutron moderators 150 impinge upon, strike,or otherwise enter the one or more neutron detectors 1200.

Although not depicted in FIGS. 13A and 13B, the entire fissile neutrondetection system 1300 may be disposed partially or completely within anexternal housing. Such may facilitate the installation of the fissileneutron detection system 1300 in an outdoor environment such as acheckpoint, port-of-entry, or similar locations where screening forfissile nuclear material may be beneficial.

The following examples pertain to embodiments that employ some or all ofthe described fissile neutron detection apparatuses, systems, andmethods described herein. The enclosed examples should not be consideredexhaustive, nor should the enclosed examples be construed to excludeother combinations of the systems, methods, and apparatuses disclosedherein and which are not specifically enumerated herein.

According to example 1 there is provided a fissile neutron detectionsystem. The fissile neutron detection system may include at least oneneutron detector. Each neutron detector may further include a bodyhaving a length, a width, and a height defining a closed chamber, thelength and the width of the chamber greater than the height of thechamber. Each neutron detector may further include at least one activematerial that emits at least one ionizing particle upon exposure tothermal neutrons, the active material disposed within the chamber; andat least one electrode. The fissile neutron detection system alsoincludes at least one neutron moderator disposed proximate the at leastone neutron detector, the at least one neutron moderator including amaterial that transitions at least a portion of high-energy incidentfissile neutrons to low-energy thermal neutrons, wherein at least 50% ofthe low-energy thermal neutrons exiting the moderator enter the at leastone detector.

Example 2 may include elements of example 1 where the chamber formed bythe body of each neutron detector may include a single, continuous,chamber.

Example 3 may include elements of example 2 where the chamber formed bythe body of each neutron detector may include a hermetically sealedchamber.

Example 4 may include elements of example 1 where the at least oneneutron detector may include a plurality of neutron detectors.

Example 5 may include elements of example 1 where the at least oneneutron moderator may include a plurality of neutron moderators.

Example 6 may include elements of example 1 where the at least oneneutron moderator may include a material that includes a minimum of 40weight percent hydrogen.

Example 7 may include elements of example 6 where the at least oneneutron moderator may include high-density polyethylene (HDPE) member.

Example 8 may include elements of example 7 where the at least oneneutron moderator may include a HDPE member having a uniform thicknessof from about 0.5 centimeters (cm) to about 10 cm.

Example 9 may include elements of example 1 where the at least oneneutron moderator comprises a number of members, each of the membershaving a uniform thickness.

Example 10 may include elements of example 1 and may additionallyinclude a voltage source conductively coupled to the at least oneelectrode in the at least one neutron detector.

Example 11 may include elements of example 1 and may additionallyinclude a number of support members disposed at intervals along at leasta portion of a length of the at least one electrode.

Example 12 may include elements of example 1 where the at least oneneutron detector may include an exterior surface having a top cover anda bottom cover separated by a sidewall having a height.

Example 13 may include elements of example 12 where the sidewallcomprises a multi-piece sidewall.

Example 14 may include elements of example 12 where the at least oneneutron detector may include a first neutron detector and a secondneutron detector; where the at least one neutron moderator may bedisposed proximate at least a portion of the exterior surface of thefirst neutron detector and at least a portion of the exterior surface ofthe second neutron detector; and where at least a portion of the atleast one neutron moderator may be disposed in a space bordered by theportion of the exterior surface of the first neutron detector and theportion of the exterior surface of the second neutron detector.

Example 15 may include elements of example 14 where the first neutrondetector may include a planar body having a planar top cover and aplanar bottom cover; where the second neutron detector may include aplanar body having a planar top cover and a planar bottom cover; andwhere the neutron moderator may include a planar member disposedproximate the top cover of the first neutron detector and the top coverof the second neutron detector.

Example 16 may include elements of example 15 where the planar top coverof the first neutron detector may have a length of from about 10centimeters (cm) to about 500 centimeters and a width of from about 10cm to about 500 cm; where the planar bottom cover of the first neutrondetector may have length of from about 10 cm to about 500 cm and a widthof from about 10 cm to about 500 cm; where the sidewall of the firstneutron detector may have a height of from about 0.5 cm to about 5 cm;where the planar top cover of the second neutron detector may have alength of from about 10 centimeters (cm) to about 500 centimeters and awidth of from about 10 cm to about 500 cm; where the planar bottom coverof the second neutron detector may have length of from about 10 cm toabout 500 cm and a width of from about 10 cm to about 500 cm; and wherethe sidewall of the second neutron detector may have a height of fromabout 0.5 cm to about 5 cm.

Example 17 may include elements of example 14 where the first neutrondetector may include an arcuate body having an arcuate top cover and anarcuate bottom cover; where the second neutron detector may include anarcuate body having an arcuate top cover and an arcuate bottom cover;and where the neutron moderator may include a constant thickness planarmember disposed proximate the top cover of the first neutron detectorand the top cover of the second neutron detector.

Example 18 may include elements of example 14 where the first neutrondetector may include an angular body having an angular top cover and anangular bottom cover; where the second neutron detector may include anangular body having an angular top cover and an angular bottom cover;and where the neutron moderator may include a constant thickness planarmember disposed proximate the top cover of the first neutron detectorand the top cover of the second neutron detector.

Example 19 may include elements of example 1 where the at least oneneutron moderator may include at least one member having an exteriorsurface; and where the at least one neutron detector may be disposedproximate at least a portion of the exterior surface of the member ofthe at least one neutron moderator.

Example 20 may include elements of example 1 where the at least oneneutron detector body may include a body having an exterior surface; andwhere the at least one neutron moderator may include at least oneexternal neutron moderator disposed proximate at least a portion of theexterior surface of the body of the at least one neutron detector.

Example 21 may include elements of example 3 where the at least oneneutron detector may include an ionizable readout gas disposed withinthe hermetically sealed chamber.

Example 22 may include elements of example 21 where the ionizablereadout gas may include at least one noble gas.

Example 23 may include elements of example 22 where the at least onenoble gas may include argon (Ar).

Example 24 may include elements of any of examples 1 through 23 wherethe at least one active material may include at least one sheet of solidactive material.

Example 25 may include elements of example 24 where the at least onesheet of active material may include at least one lithium 6 (⁶Li) sheet.

Example 26 may include elements of example 25 where each ⁶Li sheet mayinclude a ⁶Li sheet having a thickness of from about 50 micrometers (μm)to about 120 μm.

Example 27 may include elements of example 26 where each ⁶Li sheet mayinclude a ⁶Li sheet having a length and a width that exceed thethickness of the ⁶Li sheet.

Example 28 may include elements of example 27 and may additionallyinclude a support structure disposed proximate each ⁶Li sheet, thesupport structure disposed at an intermediate location within thechamber.

Example 29 may include elements of example 27 where the at least one ⁶Lisheet may be disposed proximate at least a portion of at least onesurface forming an interior of the chamber.

Example 30 may include elements of any of examples 1 through 23 wherethe at least one active material may include at least one layer ofactive material.

Example 31 may include elements of example 30 where the at least onelayer of active material may include at least one layer containing boron10 (¹⁰B).

Example 32 may include elements of example 31 where the at least onelayer containing ¹⁰B may include at least one layer of ¹⁰B disposed onat least a portion of at least one interior surface of the chamber inthe respective neutron detector.

Example 33 may include elements of any of examples 1 through 20 wherethe at least one active material comprises an active gas disposed withinthe chamber.

Example 34 may include elements of example 33 where the active gasdisposed within the chamber may include at least one gas containinghelium 3 (³He).

According to example 35, there is provided a fissile neutron detectionmethod. The fissile neutron detection method may include transitioningat least some incident fissile neutrons to thermal neutrons by passingthe incident fissile neutrons through at least one neutron moderatordisposed proximate at least one neutron detector. The at least oneneutron detector may include: a body having a length, a width, and aheight defining a closed chamber; the length and the width of thechamber greater than the height of the chamber; at least one activematerial that emits at least one ionizing particle upon exposure tothermal neutrons, the active material disposed within the chamber; andat least one electrode. The method may also include impinging at least60% of the thermal neutrons exiting the neutron moderator on the atleast one active material disposed in the chamber of the at least oneneutron detector. The method may further include generating, by the atleast one neutron detector, a current at the at least one electrode, thecurrent proportional to the number of thermal neutrons impinging on theat least one active material in the at least one neutron detector.

Example 36 may include elements of example 35 where transitioning atleast some incident fissile neutrons to thermal neutrons by passing theincident fissile neutrons through at least one neutron moderatordisposed proximate at least one neutron detector may includetransitioning at least some incident fissile neutrons to thermalneutrons by passing the incident fissile neutrons through at least oneneutron moderator proximate a plurality of neutron detectors disposedproximate at least a portion on an exterior surface of the at least oneneutron moderator.

Example 37 may include elements of example 35 where transitioning atleast some incident fissile neutrons to thermal neutrons by passing theincident fissile neutrons through at least one neutron moderatordisposed proximate at least one neutron detector may includetransitioning at least some incident fissile neutrons to thermalneutrons by passing the incident fissile neutrons through at least oneof a plurality of neutron moderators disposed proximate at least aportion on an exterior surface of the at least one neutron detector.

Example 38 may include elements of example 35 where impinging at least60% of the thermal neutrons exiting the neutron moderator on the atleast one active material disposed in the chamber of the at least oneneutron detector may include impinging at least 60% of the thermalneutrons exiting the at least one neutron moderator on at least oneactive material disposed in a hermetically sealed chamber of the atleast one neutron detector.

Example 39 may include elements of example 35 and may additionallyinclude generating, at least one signal proportional to at least one of:the number of thermal neutrons impinging on the at least one activematerial in the at least one neutron detector or the rate of thermalneutron impingements on the at least one active material in the at leastone neutron detector.

Example 40 may include elements of example 35 where transitioning atleast some incident fissile neutrons to thermal neutrons by passing theincident fissile neutrons through at least one neutron moderatordisposed proximate at least one neutron detector may includetransitioning at least some incident fissile neutrons to thermalneutrons by passing the incident fissile neutrons through at least oneneutron moderator that includes a minimum of 40 weight percent hydrogen.

Example 41 may include elements of example 40 where transitioning atleast some incident fissile neutrons to thermal neutrons by passing theincident fissile neutrons through at least one neutron moderator thatincludes a minimum of 40 weight percent hydrogen may includetransitioning at least some incident fissile neutrons to thermalneutrons by passing the incident fissile neutrons through at least oneneutron moderator that includes a material containing a high densitypolyethylene (HDPE).

Example 42 may include elements of example 41 where transitioning atleast some incident fissile neutrons to thermal neutrons by passing theincident fissile neutrons through at least one neutron moderator thatincludes a material containing a high density polyethylene (HDPE) mayinclude transitioning at least some incident fissile neutrons to thermalneutrons by passing the incident fissile neutrons through at least oneneutron moderator that includes HDPE having a thickness of from about0.5 centimeters (cm) to about 10 cm.

Example 43 may include elements of example 35 and may additionallyinclude at least partially encapsulating at least a portion of the atleast one neutron detector and at least a portion of the at least oneneutron moderator in an external neutron moderator.

Example 44 may include elements of example 43 where at least partiallyencapsulating at least a portion of the at least one neutron detectorand at least a portion of the at least one neutron moderator in anexternal neutron moderator may include at least partially encapsulatingat least a portion of the at least one neutron detector and at least aportion of the at least one neutron moderator in a material thatincludes a minimum of 40 weight percent hydrogen.

Example 45 may include elements of example 44 where at least partiallyencapsulating at least a portion of the at least one neutron detectorand at least a portion of the at least one neutron moderator in amaterial that includes a minimum of 40 weight percent hydrogen mayinclude at least partially encapsulating at least a portion of the atleast one neutron detector and at least a portion of the at least oneneutron moderator in a material that includes high-density polyethylene(HDPE).

Example 46 may include elements of any of examples 35 through 45, wheregenerating, by the at least one neutron detector, a current at the atleast one electrode, the current proportional to the number of thermalneutrons impinging on the at least one active material in the at leastone neutron detector may include, for each thermal neutron impinging onthe at least one active material in the at least one neutron detector,generating at least one ionizing particle by at least one sheet ofactive material; ionizing, by the at least one ionizing particle, areadout gas disposed within the chamber of the at least one neutrondetector; maintaining the at least one electrode disposed in the chamberof the at least one neutron detector at a voltage that differs from avoltage of the at least one sheet of active material; causing, by theionized readout gas, a flow of charged particles away from the at leastone electrode; and causing a current at the electrode by the flow ofcharged particles, the current proportional to the number of thermalneutrons impinging on the at least one sheet of active material disposedin the chamber of the at least one neutron detector.

Example 47 may include elements of example 46 where ionizing a readoutgas disposed within the chamber of the at least one neutron detector mayinclude ionizing, by the at least one ionizing particle, a noble readoutgas disposed within the chamber of the at least one neutron detector.

Example 48 may include elements of example 46 where maintaining the atleast one electrode disposed in the chamber of the at least one neutrondetector at a voltage that differs from a voltage of the at least onesheet of active material may include biasing the at least one electrodeto a potential of at least +100 volts (V) measured with respect to thepotential of the at least one sheet of active material.

Example 49 may include elements of example 46 where generating at leastone ionizing particle by at least one sheet of active material mayinclude generating the at least one ionizing particle by at least onesolid sheet of active material disposed within the chamber of the atleast one neutron detector.

Example 50 may include elements of example 49 where generating the atleast one ionizing particle by at least one solid sheet of activematerial disposed within the chamber of the at least one neutrondetector may include generating at least one ionizing particle by atleast one solid sheet of active material comprising at least one lithium6 (⁶Li) sheet disposed within the chamber of the at least one neutrondetector.

Example 51 may include elements of example 50 where generating at leastone ionizing particle by at least one solid sheet of active materialcomprising at least one lithium 6 (⁶Li) sheet disposed within thechamber of the at least one neutron detector may include generating atleast one ionizing particle by at least one solid sheet of activematerial comprising at least one ⁶Li sheet having a thickness of fromabout 50 micrometers to about 100 micrometers disposed within thechamber of the at least one neutron detector.

Example 52 may include elements of example 51 where generating at leastone ionizing particle by at least one solid sheet of active materialcomprising at least one 6Li sheet having a thickness of from about 50micrometers to about 100 micrometers disposed within the chamber of theat least one neutron detector may include generating at least oneionizing particle by at least one solid sheet of active materialcomprising at least one ⁶Li sheet disposed within the chamber of the atleast one neutron detector, the at least one sheet of ⁶Li comprising atleast one of: a single ⁶Li sheet proximate a support structure andpositioned at an intermediate point within the chamber of the at leastone neutron detector; at least one ⁶Li sheet disposed proximate at leasta portion of at least one wall forming at least a portion of the chamberof the at least one neutron detector; or a number of spaced ⁶Li sheetsproximate a support structure and positioned at an intermediate pointwithin the chamber of the at least one neutron detector.

Example 53 may include elements of any of claims 35 through 45 wheregenerating, by the at least one neutron detector, a current at the atleast one electrode, the current proportional to the number of thermalneutrons impinging on the at least one active material in the at leastone neutron detector may include, for each thermal neutron impinging onthe at least one active material in the at least one neutron detector,generating at least one ionizing particle by at least one layer ofactive material disposed within the chamber of the at least one neutrondetector; ionizing, by the at least one ionizing particle, a readout gasdisposed within the chamber of the at least one neutron detector;maintaining the at least one electrode disposed in the chamber of the atleast one neutron detector at a voltage that differs from a voltage ofthe at least one layer of active material; causing, by the ionizedreadout gas, a flow of charged particles away from the at least oneelectrode; and causing a current at the electrode by the flow of chargedparticles, the current proportional to the number of thermal neutronsimpinging on the at least one layer of active material disposed in thechamber of the at least one neutron detector.

Example 54 may include elements of example 53 where generating at leastone ionizing particle by at least one layer of active material disposedwithin the chamber of the at least one neutron detector may includegenerating at least one ionizing particle by at least one layer ofactive material comprising at least one layer containing ¹⁰B disposedwithin the chamber of the at least one neutron detector.

Example 55 may include elements of example 54 where generating at leastone ionizing particle by at least one layer of active materialcomprising at least one layer containing ¹⁰B disposed within the chamberof the at least one neutron detector may include generating at least oneionizing particle by at least one layer of active material comprising atleast one layer containing ¹⁰B disposed proximate at least a portion ofat least one wall forming at least a portion of the chamber of the atleast one neutron detector.

Example 56 may include elements of example 53 where ionizing a readoutgas disposed within the chamber of the at least one neutron detector mayinclude ionizing, by the at least one ionizing particle, a noble readoutgas disposed within the chamber of the at least one neutron detector.

Example 57 may include elements of examples 35 through 45 wheregenerating, by the at least one neutron detector, a current at the atleast one electrode, the current proportional to the number of thermalneutrons impinging on the at least one active material in the at leastone neutron detector may include, for each thermal neutron impinging onthe at least one active material in the at least one neutron detector,generating at least one ionizing particle by at least one active gasdisposed within the chamber of the at least one neutron detector;maintaining the at least one electrode disposed in the chamber of the atleast one neutron detector at a potential greater than the at least oneactive gas; causing, by the ionized readout gas, a flow of chargedparticles away from the at least one electrode; and causing a current atthe electrode by the flow of charged particles, the current proportionalto the number of thermal neutrons impinging on the at least one layer ofactive material disposed in the chamber of the at least one neutrondetector.

Example 58 may include elements of example 57 where generating at leastone ionizing particle by at least one active gas disposed within thechamber of the at least one neutron detector may include generating theat least one ionizing particle by at least one active gas that includeshelium 3 (³He), the at least one active gas disposed within the chamberof the at least one neutron detector.

The terms and expressions which have been employed herein are used asterms of description and not of limitation, and there is no intention,in the use of such terms and expressions, of excluding any equivalentsof the features shown and described (or portions thereof), and it isrecognized that various modifications are possible within the scope ofthe claims. Accordingly, the claims are intended to cover all suchequivalents.

1. A fissile neutron detection system, comprising: at least a firstthermal neutron detector and a second thermal neutron detector in aspaced apart confronting relationship as part of an at least generallyplanar layered arrangement such that each thermal neutron detector atleast generally passes incident fissile neutrons therethrough and eachthermal neutron detector including: a body having a shape includinglateral extents and a height defining a closed chamber that contains areadout gas such that the readout gas is disposed within said closedchamber, the lateral extents of the body greater than the height of thebody so as to define first and second major surfaces; at least onearrangement of active sheet material, as part of the generally planarlayered arrangement, that emits ionizing particles upon exposure tothermal neutrons and the ionizing particles initiate an avalanche ofions, within said readout gas, to produce a current flow, thearrangement of active sheet material disposed within the chamber; atleast one electrode within the chamber to generate an electrical outputsignal responsive to the current flow; and at least one neutronmoderator including a pair of opposing major sides, the neutronmoderator positioned between the first thermal neutron detector and thesecond thermal neutron detector, as another part of the at leastgenerally planar layered arrangement, such that one major surface of thefirst thermal neutron detector confronts one major side of the moderatorand another major surface of the second neutron detector confronts theother, opposing major side of the moderator for converting the incidentfissile neutrons, passing through one of the first and second thermalneutron detectors without detection, to said thermal neutrons responsiveto the neutron moderator transitioning at least a portion of thehigh-energy incident fissile neutrons to low-energy thermal neutrons,wherein a majority of the low-energy thermal neutrons exiting themoderator, originating from the incident fissile neutrons that initiallypass undetected through either one of the first thermal neutron detectorand the second thermal neutron detector, enter one or the other of thefirst and second neutron detectors for subsequent detection of at leastsome of the thermal neutrons by one of the first thermal neutrondetector and the second thermal neutron detector.
 2. The fissile neutrondetection system of claim 1: wherein said active sheet material includesat least one of (i) lithium and (ii) boron.
 3. The fissile neutrondetection system of claim 1: wherein each of the first and secondthermal neutron detectors is configured to support a voltage biasbetween the arrangement of active sheet material and the electrode, andsaid ionization, initiated by said ionizing particles, is multiplied inpart by said voltage bias acting on said ions to cause an avalanche ofions as part of said current flow.
 4. The fissile neutron detectionsystem of claim 1: wherein for each one of the first and second thermalneutron detectors, the arrangement of active sheet material thereofsubstantially overlays an adjacent one of the first and second opposingmajor sides of the neutron moderator.
 5. The fissile neutron detectionsystem of claim 1: wherein the first and second thermal neutrondetectors surround at least a majority of the neutron moderator in saidlayered structure such that, after exiting the neutron moderator, atleast 60% of said thermal neutrons impinge upon an active area of thethermal neutron detector that is made up of the arrangements of activesheet material of the first and second thermal neutron detectors.
 6. Thefissile neutron detection system of claim 1: wherein at least one of theaforerecited first and second thermal neutron detectors comprises aplurality of thermal neutron detectors transversely aligned inside-by-side relationships with one another to form one layer of the atleast generally planar layered arrangement, such that each one of theplurality of side-by-side thermal neutron detectors is in closeproximity to the adjacent one of the first and second major sides of theneutron moderator.
 7. The fissile neutron detection system of claim 1:wherein each closed chamber includes at least one cover, at least onewall and a joint therebetween, as parts of said closed chamber, and saidjoint is hermetically sealed by one or more elastomeric materials thatare disposed within said joint.
 8. The fissile neutron detection systemof claim 7: wherein at least one of said elastomeric materials comprisespolyisobutylene.
 9. The fissile neutron detection system of claim 1:wherein the readout gas includes Argon.
 10. The fissile neutrondetection system of claim 7, further comprising: at least one isolatorsupporting a portion of said electrode for extending through one of saidparts of the closed chamber, and the isolator is configured to (i)electrically isolate said portion of the electrode from the closedchamber, and (ii) hermetically seal about the electrode.
 11. The fissileneutron detection system of claim 1 wherein the aforerecited neutronmoderator serves as a first neutron moderator, and the fissile neutrondetection system further comprises at least a second neutron moderator,as an additional part of said layered structure, in a confrontingproximate relationship with said first thermal neutron detector with thefirst thermal neutron detector interposed between the first neutronmoderator and the second neutron moderator.
 12. The fissile neutrondetection system of claim 11: wherein the first neutron moderatorincludes a thickness that is (i) less than any lateral extent thereof,and (ii) greater than a thickness of the second neutron moderator. 13.The fissile neutron detection system of claim 11, further comprising: athird neutron moderator in a confronting proximate relationship with thesecond thermal neutron detector, with the second thermal neutrondetector interposed between the first neutron moderator and the thirdneutron moderator.
 14. The fissile neutron detection system of claim 1:wherein at least one of the aforerecited first and second thermalneutron detectors comprises a plurality of thermal neutron detectorstransversely aligned in side-by-side relationships with one another toform one layer of the at least generally planar layered arrangement,such that each one of the plurality of side-by-side thermal neutrondetectors is in close proximity to the adjacent one of the first andsecond major sides of the neutron moderator and any given one of saidplurality of side-by-side thermal neutron detectors includes a detectorchamber that supports at least one layer of an active sheet material andthe layer of active sheet material spans across at least a majority ofthe lateral extents of the given thermal neutron detector. 15-58.(canceled)
 59. A fissile neutron detection system, comprising: a thermalneutron detection arrangement including at least a first thermal neutrondetector arrangement in a spaced apart confronting relationship with asecond thermal neutron detector arrangement, forming part of an at leastgenerally planar layered arrangement, with each thermal neutron detectorarrangement configured (i) to generate an electrical output signalresponsive to thermal neutrons interacting therewith, (ii) to at leastgenerally pass incident fissile neutrons therethrough without detection,and (iii) having a thickness that is less than any given lateral extentthereof; and a moderator arrangement, forming another part of thegenerally planar layered arrangement and defining first and secondoutwardly facing opposing major surfaces, interposed between the firstthermal neutron detector arrangement and the second thermal neutrondetector arrangement to receive said incident fissile neutrons, witheach incident fissile neutron entering the moderator arrangement throughone or the other of the first and second opposing surfaces forconversion of the fissile neutrons into thermal neutrons that aredetectable by the first and second thermal neutron detectorarrangements, and the first outwardly facing major surface of themoderator arrangement faces the first thermal neutron detectorarrangement with at least a majority of the first outwardly facing majorsurface proximate to the first thermal neutron detector arrangement andthe second outwardly facing major surface of the moderator sheetmaterial faces the second thermal neutron detector arrangement with atleast a majority of the second external surface proximate to the secondthermal neutron detector arrangement, wherein each of the first andsecond outwardly facing major surfaces of the moderator arrangement isin sufficiently close proximity to a respective one of the first andsecond detector arrangements, such that the incident fissile neutronsfirst pass through one or the other of said first and second thermalneutron detector arrangements before entering into said moderatorarrangement, and at least a majority of said thermal neutrons thatoriginate from the incident fissile neutrons enter one or the other ofsaid first and second thermal neutron detector arrangements forsubsequent detection of at least some of the thermal neutrons by saidfirst and second thermal neutron detector arrangements.
 60. The fissileneutron detection system of claim 59 wherein each one of the first andsecond thermal neutron detector arrangements includes, as part of saidlayered structure, at least one arrangement of active sheet material andone or more electrodes with a readout gas disposed between thearrangement of active sheet material and the electrodes, and at leastsome of the majority of thermal neutrons entering the first and secondthermal neutron detector arrangements are received by the arrangement ofactive sheet material of each of the first and second thermal neutrondetector arrangements and converted, by impact with the arrangement ofactive sheet material, into charged daughter particles having sufficientkinetic energy to at least partially ionize said readout gas to cause acurrent flow that is captured by said electrodes, and collected as saidelectrical signal.
 61. The fissile neutron detection system of claim 60wherein said active sheet material includes at least one of (i) lithiumand (ii) boron.
 62. The fissile neutron detection system of claim 60wherein for each one of the first and second thermal neutron detectorarrangements, the arrangement of active sheet material thereofsubstantially overlays an adjacent one of the first and second outwardlyfacing opposing major surfaces of the moderator arrangement.
 63. Thefissile neutron detection system of claim 60 wherein the first andsecond thermal neutron detector arrangements surround at least amajority of the moderator arrangement in said layered structure suchthat, after exiting the moderator, at least 60% of said thermal neutronsimpinge upon an active area of the thermal neutron detector arrangementthat is made up of the arrangements of active sheet material of thefirst and second thermal neutron detectors.
 64. The fissile neutrondetection system of claim 60 wherein at least one of the first andsecond thermal neutron detector arrangements includes a detector chamberthat is configured to support said arrangement of active sheet material,and the detector chamber is hermetically sealed to contain said readoutgas hermetically sealed therein.
 65. The fissile neutron detectionsystem of claim 64 wherein the detector chamber includes at least onecover and at least one wall, as parts of said detector chamber, with ajoint therebetween, and said joint is hermetically sealed, at least inpart by one or more elastomeric materials that are disposed within saidjoint.
 66. The fissile neutron detection system of claim 65 wherein atleast one of said elastomeric materials comprises polyisobutylene. 67.The fissile neutron detection system of claim 65 including at least oneisolator supporting at least one of said electrodes for extendingthrough one of said parts of said detector chamber, and the isolator isconfigured to (i) electrically isolate at least one of said electrodesfrom said detector chamber, and (ii) hermetically seal the chamber abouta portion of that electrode.
 68. The fissile neutron detection system ofclaim 60 wherein said readout gas includes Argon.
 69. The fissileneutron detection system of claim 59 wherein each of the first andsecond thermal neutron detector arrangements is configured to support avoltage bias between the arrangement of active sheet material and atleast one of the electrodes, and said current flow is initiated by saidionization and multiplied in part by said voltage bias acting on saidions to cause an avalanche of ions as part of said current flow.
 70. Thefissile neutron detection system of claim 59 wherein at least one ofsaid first and second thermal neutron detector arrangements includes aplurality of thermal neutron detectors transversely aligned inside-by-side relationships with one another to form one layer of the atleast generally planar layered arrangement, such that each one of theplurality of side-by-side thermal neutron detectors is in closeproximity to the adjacent one of the first and second outwardly facingopposing surfaces.
 71. The fissile neutron detector system of claim 70wherein any given one of said plurality of side-by-side thermal neutrondetectors includes a detector chamber that supports at least one layerof an active sheet material and the layer of active sheet material spansacross at least a majority of lateral extents of the given thermalneutron detector.
 72. The fissile neutron detection system of claim 59wherein the aforerecited moderator arrangement serves as a firstmoderator arrangement, and the fissile neutron detector system furthercomprises at least a second moderator arrangement, as an additional partof said layered structure, in a confronting proximate relationship withsaid first thermal neutron detector arrangement with the first thermalneutron detector arrangement interposed between the first and secondmoderator arrangements.
 73. The fissile neutron detection system ofclaim 72 wherein said first moderator arrangement includes a thicknessthat is (i) less than any lateral extent thereof, and (ii) greater thana different thickness of the second moderator arrangement.
 74. Thefissile neutron detection system of claim 72 further comprising a thirdmoderator arrangement in a confronting proximate relationship with thesecond thermal neutron detector arrangement with the second neutrondetector arrangement interposed between the first and third moderatorarrangements.
 75. A method for producing a fissile neutron detectionsystem, comprising: configuring a thermal neutron detection arrangementto include at least a first thermal neutron detector arrangement in aspaced apart confronting relationship with a second thermal neutrondetector arrangement to form part of an at least generally planarlayered arrangement, with each thermal neutron detector arrangementconfigured (i) to generate an electrical output signal responsive tothermal neutrons interacting therewith, (ii) to at least generally passincident fissile neutrons therethrough without detection, and (iii)having a thickness that is less than any given lateral extent thereof;and interposing a moderator arrangement, as another part of thegenerally planar layered arrangement which defines first and secondoutwardly facing opposing major surfaces, between the first thermalneutron detector arrangement and the second thermal neutron detectorarrangement to receive said incident fissile neutrons such that eachincident fissile neutron enters the moderator arrangement through one orthe other of the first and second opposing surfaces for conversion ofthe fissile neutrons into thermal neutrons that are detectable by thefirst and second thermal neutron detector arrangements, and interposingcauses the first outwardly facing major surface of the moderatorarrangement to face the first thermal neutron detector arrangement withat least a majority of the first outwardly facing major surfaceproximate to the first thermal neutron detector arrangement and thesecond outwardly facing major surface of the moderator sheet material toface the second thermal neutron detector arrangement with at least amajority of the second external surface proximate to the second thermalneutron detector arrangement, wherein each of the first and secondexternal outwardly facing major surfaces of the moderator arrangementare in sufficiently close proximity to a respective one of the first andsecond detector arrangements such that the incident fissile neutronsfirst pass through one or the other of said first and second thermalneutron detector arrangements before entering into said moderatorarrangement, and at least a majority of said thermal neutrons thatoriginate from the incident fissile neutrons enter one or the other ofsaid first and second thermal neutron detector arrangements forsubsequent detection of at least some of the thermal neutrons by saidfirst and second thermal neutron detector arrangements.